Arnold Zwicky (Language Log) has an excellent post:
Philip Gourevitch's "The State of Sarah Palin" (New Yorker, 22 September, p. 66-7) quotes from an interview with the vice-presidential candidate:
"We're not just gonna concede to three big oil companies of this monopoly --- Exxon, B.P., ConocoPhillips --- and beg them to do this [build a natural gas pipeline] for Alaska," Palin told me last month in Juneau. "We're gonna say, 'O.K., this is so economic that we don't have to incentivize you to build this. In fact, this has got to be a mutually beneficial partnership here as we build it. We're gonna lay out Alaska's must-haves. Parameters are gonna be set, rules are gonna be laid out, a law will encompass what it is that Alaska needs to protect our sovereignty, to insure it's jobs first for Alaskans, and in-state use of gas'" --- her list went on.
What stands out here — for a linguist, anyway — is the five occurrences of the spelling gonna for written standard going to. I'll take Gourevitch's word that this is the way Palin pronounced the expression, but why did he transcribe it that way? ....
First point: gonna is an entirely standard, though informal variant of going to, at least in American English.... Instances of gonna from standard-English American speakers in relaxed contexts are all over the place, and it's not hard to find the occasional instance from such speakers (even prestigious ones) in formal contexts. Normally we'd expect such occurrences of gonna to to be represented as going to in print.
Fourth point: ... using non-standard spellings like gonna for standard (but informal) phonological variants paints the speaker as folksy, rustic, etc.... The writer thus covertly injects a social judgment about the speaker into what is framed as a report of an interview about experiences and opinions. In the pages of the New Yorker, N variants convey a negative judgment (because the magazine's readers are likely to hold to the belief that the N variants are, if not simply non-standard, that is, "incorrect", then at least rough, "hick", variants). In other publications, N variants might be understood differently....
The rest of the post is much worth reading as well (as is characteristic of Language Log, which does a great job of applying its authors' scholarly and professional knowledge to lay topics).
My quick thought on the situation: People have both personal and regional variants in their pronunciations, such as the Southern "Ah" for "I," some people's preference for "cyoopon," particular people's lingering foreign accents (like, er, maybe mine), and the like. The speakers are still using the same words as everyone else — they're just pronouncing them slightly differently.
It seems to me that written quotes ought to capture the words used, and not the pronunciation. We wouldn't normally quote a lisper as saying, "We're going to thay, 'OK, thith ..." (at least unless the lisp is the focus of the story). We wouldn't quote a Southerner as saying "Ah buhlieve ...." We shouldn't quote someone as saying "Febyooary" when he means February.
Likewise, it seems to me that "going to" should be quoted as "going to" even when it acoustically resembles "gonna," at least setting aside unusual circumstances (for instance, if the argument is generally about the speaker's deliberately folksy pronunciations, something Zwicky reports this article is not). Such phonetic spelling strikes me as sometimes distracting to readers. And it strikes me as generally unfair to the speaker, whose regional background, speech impediment, or foreignness the phonetic spelling unduly emphasizes.
UPDATE: Here's the entire New Yorker piece.