In the past few days I?ve seen two different Facebook profile pictures with ?I am a Mormon? written in Deseret. The happy confluence of Mormonness and nerdiness in these images makes me happy. Further, even though I know very little about Deseret or its mechanics, these images also give me entrée to talk about two of the (many) reasons Deseret failed to catch on in Mormonism or anywhere else.
Notice that the texts are different, as shown below. 
I don?t know why Nate or Enid used ?I am a Mormon? rather than ?I?m a Mormon? like the marketing campaign. Enid?s version of ?Marmon? sounds like it might have spent some time in ?Spanish Fark.? It also matches the rendering of ?Mormon? in the 1869 edition of the Book of Mormon in Deseret.  Nate?s indefinite article (the ?a? in ?a Mormon?) matches the indefinite articles on the title page (?a remnant?) and in ?The Testimony of the Eight Witnesses? (?a surety?) in the 1869 BoM.
Below I provide two alternate renderings:
It?s only ?almost? because Deseret can?t quite get the indefinite article.  Note that the contracted ?I?m? has no apostrophe. Since Deseret is phonetic I don?t have to indicate possession orthographically: I can just write it how it sounds to me.
And therein lies two rubs. People speak and hear sounds differently across time and space and written languages have different rules and norms than spoken languages. Above are four versions of the same idea, all at least partially plausible. I couldn?t swear that any one is ?the correct one? but it is sort of historically ?authentic? that there should be more than one rendering of the same exact sentence.
For the bonus round, two more versions and some interesting reads:
Neil Alexander Walker, ?A Complete Guide to Reading and Writing the Deseret Alphabet,? 2005.
Frederick M. Huchel, ?The Deseret Alphabet as an Aid in Pronouncing Book of Mormon Names,? Journal of Book of Mormon Studies 9.1 (2000): 58?59, 79.
M Scott Reynolds, The Deseret Alphabet Portal, 2010.
 The Deseret font I use throughout is ?Zarahemla Bee,? by Joshua Erickson, available, free, here.
 I haven?t asked Nate, but his version seems to be machine-generated. Deseret is a self-consciously phonetic script so different sounds have different characters, at least as far as you can with only thirty-eight characters. In Nate?s sample each letter in written English is replaced by a single character in Deseret. Thus, there are five vowel sounds represented by three characters (i, a, and o); in a more precisely rendered Deseret, we?d expect five different vowel symbols for five different sounds, like we see in Enid?s version.
 I don?t detect any hint of an ?a? when I say ?Mormon? and there is usually no difference in how I pronounce ?more,? ?Moore,? and the first syllable of ?Mormon.? But, I can only do ?almost? because I frequently use a ?schwa? for the indefinite article. That is, I say ?uh mohr mun.? Even though Deseret attempted to be phonetic, and the schwa is and was one of the most common vowel sounds in American Englishes, Deseret does not have a symbol for schwa.