From Mark Liberman over at Language Log, some musings on "eye dialect" (neat term, that)
Eye dialect being, the use of spellings to indicate a spoken dialect and pronunciation.
In its simplest, it's things like spellin' instead of spelling.
At it's most extreme, was as "wuz" in order to indicate that the speaker is stupid (OK, maybe not stupid, but rough and unlettered)--even though, well, even the most erudite of folks pronounces was as "wuz."
More nuances and expertise in Mr. Liberman's post, and in the comments.