As I mentioned in the prospectus I posted, I see both striking resemblances between Mormonism and late Neoplatonism and important influences of late Neoplatonism on the history of Christianity that need to be explored. My committee balked at linking Mormonism to late Neoplatonism and wanted further proof. So I?ve been doing some research.
Why I?ve wanted to stick with late Neoplatonism as illuminating early Mormonism is because I saw some very useful paradigms. At UCSB, under the encouragement of my adviser, I wanted to learn more about early Christianity with an eye toward understanding Brooke?s Refiners Fire. I audited a class from Beth DePalma Digeser on early Christianity who made it clear that she had a lot of expertise in the overlap between early Christianity and Neoplatonism. So I went by her office and asked if I could do a field exam with her on these topics. Later when I was ready to get going, she handed me a list of books. The first one I read was Gregory Shaw?s book on Iamblichus. The approach I had been taking on the Smith?s early ?magical? practices was to think of them in terms of pre-Reformation holdovers but ultimately to try to understand their ?religious? components. ?Theurgy? seemed very helpful in this task. The goal was to do rituals to come into the presence of God and become divinized, which I saw as infused throughout Mormonism. This was also central to rituals usually called ?ritual magic? or ?angel conjuring? that persisted throughout the history of Christianity and that the Smith?s may have been engaging in.
So I really liked the term, but the more I thought about it, I wanted to get at what Smith?s central goals were. Smith did create rituals and means for others to come into the presence of God but he had done so himself in the First Vision. Afterwards he had additional goals, which seemed to be creating Zion, the perfect society. The central motif seemed to be the City of Enoch. Failing at that (the expulsion from Missouri) JS sought to bind his close friends to him to create a nucleus of heaven. So I looked for medieval prototypes and proceeded with that.
But as I was reading medieval stuff I kept running into the fact that Christians who were in to Plato promoted Mormon-looking ideas: heavenly mother, deification, pre-existence, etc. So after I was done with Prof. Digeser?s list, I told her of this trend that I had found and that I wanted to know more about Plato and his influence on the early Middle Ages. So she suggested that I read Dominic O?Meara?s Platonopolis. O?Meara argues that it had been assumed that the Neoplatonists were not political but this was not the case, particularly among the late Neoplatonists. The goal of the late Neoplatonists, argues O?Meara, was to become divinized through theurgy, but then, if one had philanthropy, he or she would go back into the cave to enlighten his or her fellow beings. The ultimate goal of the late Neoplatonists was to create Plato?s Republic, led by the Philosopher King. Yet the late Neoplatonists despaired of creating the Republic on earth and came to believe that it only truly existed in heaven?the city of the gods. Proclus conceived of three heavens. The Republic, where everything was shared, was the highest state. Lower heavenly cities also exited, based on Plato?s other writings (more on that in a bit). 
This seemed to match up remarkably well with what I saw in JS. So when my committee demanded that I either find late Neoplatonic influence on JS or drop it, I though I?d do some more digging.
1) First off, the late Neoplaotnists (Iamblichus and Proclus) do some interesting things with Platonism. Not to get into it all, but previous commenters wondered about Platonism and materialism. With the introduction of theurgy into Platonism by Iamblichus the physical became more important: physical rituals became a way to access The One.  The body was also a vehicle for the soul?s elevation.
2) Proclus was quite influential on the development of Christianity. First through Pseudo-Dionysius, because he claimed to be a convert of St. Paul, his writings were deemed semi-scriptural in the Middle Ages. Pseudo-Dionysius sought to merge Proclus with Christianity and may have been a devotee of Proclus. Another Christian devotee of Proclus was Boethius who quoted extensively from Proclus in his influential Consolation of Philosophy.
3) Other Proclus works popped up in the Middle Ages: The Book of Causes circulated in the Middle Ages which was a summary of his important Elements of Theology but attributed to Aristotle until the Elements were translated into Latin in 1268. This sparked an interest in such things and influenced Meister Eckhart who had a major influence on late medieval mysticism. He taught things like the idea that humans had uncreated souls and could become gods and even that God had become God. 
4) Brian Copenhaver argues that scholars have misunderstood Ficino and his Hermetic revival based on Ficino?s own misunderstandings. The belief that the Hermetic writings were ancient, dating back to Moses, dated back to antiquity. That is why de Medici instructed Ficino to translate the Corpus Hermeticum before Plato: the Corpus was deemed to be the source of Plato?s ideas. As a result, Ficino believed that Platonism and late Neoplatonism came out of Hermeticism. It was believed that Proclus, who Ficino also translated and was heavily influenced by, was simply the best expositor of Hermeticism. Thus the things that Renaissance Hermeticists and later scholars like Francis Yates called Hermeticism were largely the ideas of Proclus. Much of what Renaissance Platonists and Francis Yates called Hermeticism are not in the Corpus Hermiticum but are instead found in Proclus. 
5) The Cambridge Platonists were also into Neoplatonism but seemingly focused more on Plotinus than Proclus.
6) The main translator of Plato into English was Thomas Taylor who did so around 1800. Taylor was a Proclus devotee and his commentary on Plato intermixed in his translation was Proclus heavy. Says Jay Bregman, ?Taylor?s work suggests that he was, metaphorically at least, a ?reincarnation? of Proclus. His translations of all Greek philosophers evoke the Greek style of Proclus transliterated into English, as it were. New England intellectuals, therefore, read Plato, and Plotinus as well, through ?Procline spectacles.??  Taylor went so far as to name his son Thomas Proclus Taylor.
7) The biggest contemporary devotees of Proclus among JS?s contemporaries were the Transcendentalists. Emerson wrote in his journal in 1843 that ?I read Proclus for my opium.? 
8 Emerson is a little late to be influential on JS but it is possible that the things that influenced Emerson could have influenced JS. Harvard got a copy of Taylor?s translations of Plato in 1820 and Emerson started reading them in 1828. Emerson did not start seriously reading Neoplatonic texts until after 1837, but he was aware of their ideas from Taylor?s translation of Plato and from reading Ralph Cudworth?s (the Cambridge Platonist) True Intellectual System. Other possible sources include the Romantic poets Coleridge (and his Biographia Literaria chapter 12) and Wordsworth, who were also influenced by Neoplatonism. 
9) There are striking similarities in things like DC 88, Book of Abraham, the interest in Thomas Dick. 
10) Where the term Telestial comes from has long eluded Mormon scholars. Late Neoplatonism offers a possibility. First, Proclus had a three-tired heaven. Second, the word ?telestic? was used by the late Neoplatonists as a synonym for Theurgy. Another late Neoplatonist name Hierocles came up with a triadic typology for virtues: the highest was the philosophical, the second was the political, and the third was the telestic. That is, for Hierocles, one started with telestic rites and then moved upward in order to elevate the soul to become one with God.  The fact that telestic was third in his typology is striking considering that late Neoplatoists also had a three-tired heaven. Some quick Google searching said that Hierocles?s writings were around in the Middle Ages and were in Emerson?s library.
The word itself doesn?t have much to do with the usage of Telestial but Celestial and Terrestrial are also ill-fitting terms: all the kingdoms are ?heavenly? and none of them are ?earthy.? The terms are only used to designate their rank: Celestial best, Terrestrial second, Telestial third.
11) If late Neoplatonism was the source for Telestial, then it would suggest familiarity with late Neoplatonism among JS?s inner circle. So the question is, what did Rigdon (Pratts, Phelps) read?
 Dominic J. O?Meara. Platonopolis: Platonic Political Philosophy in Late Antiquity (Oxford: Clarendon Press, 2003), 95-96.
 Andrew Itter, ?Psuedo-Dionysian Soteriology and Its Transformation of Neoplatonism? Colloquium 32/1 (2000), 77.
 I heard a paper given on this topic at last years Catholic history meeting held at AHA.
 Brian Copenhaver, ?Hermes Trismegistus, Proclus, and the Question of a Philosophy of Magic in the Renaissance,? in Hermeticism and the Renaissance: Intellectual History and the Occult in Early Modern Europe, ed. by Ingrid Merkel and Allen G. Debus (Washington: Folger Shakespeare Library, 1988), 79-110; and D. P. Walker, The Ancient Theology: Studies in Christian Platonism from the Fifteenth to the Eighteenth Century (Ithaca: Cornell University Press, 1972).
 Jay Bregman ?Proclus Americanus,? Late Antique Epistemology: Other Ways to Truth, Ed by Panayiota Vassilopoulou and Stephen R. L. Clark (Houndsmill, U.K.: Palgrave Macmillan, 2009), 228.
 Bregman, ?Proclus Americanus,? 229.
 Kenneth Walter Cameron, Young Emerson?s Transendental Vision: An Exposition of His World View with an Analysis of the Structure, Backgrounds, and Meaning of Nature (1836) (Hatford: Transcendental Books, 1971), 37-46.
 Sam Brown?s forthcoming article does a good job of pointing out similarities. I would only add that the since the chain of being was only one aspect of Neoplatonism, there remains much more to be studied. Samuel Brown, ?The Early Mormon Chain of Belonging,? Dialogue, forthcoming.
 O?Meara, Platonopolis, 129-130.