Dee's already made the point I wanted to make when I first read Kate's question about punctuation. A first person narrator is a stylistic device - a method of telling a story. In most cases there is no intention to give the impression that the narrator is 'writing' the story. Admittedly, this is exactly what is happening in my novel 'No Mystics' (shameless plug - Chapter 1 available in an archive near you), where the narrator is sometimes 'making notes' for his biography, which is being written by someone else, but is also sometimes describing, in present tense, what is happening around him. It has never once occurred to me that punctuation should match the character. Fortunately in this case it does anyway, as he's an articulate bloke whose education - grammar school but didn't finish sixth form - matches mine.
Personally, I wouldn't risk using poor punctuation to 'describe' a character, as I'm fairly certain no reader would ever get the point and it would only reflect badly on my command of grammar. I don't try to write in dialect, either. Accents can be hard enough to understand aurally; interpreted into phonetics they're impossible - and horrible.
I've not been around this site very long, so I don't know if anyone's ever done this, but an interesting assignment set by my Prose Fiction tutor when I did my degree was to write a short story in first person from the point of view of an inarticulate character; someone with an extremely restricted command of language. I remember writing mine very quickly then revising the punctuation to pace the story. As it was literally a character 'saying' everything that was happening, I took out all the commas, as they seemed to express an element of consideration - pausing to think - which didn't suit the narrative. Although it's certainly rather 'out of character' for me, as I'm no short story writer (didn't have a choice during the degree, though), I'll try to hunt it out and post it up - it's only 500 words or so. It's certainly an interesting challenge!
Julie
<Added>If anyone's interested, the story's called 'Mammie's Boy' and it should be here:
http://www.writewords.org.uk/archive/9765.asp