Someone once paid me the compliment of calling me a writer even though he hadn't read any of my stories. What had he laid eyes upon? Exactly one of my emails.
Despite my comments about text-sex-ing (how does one make that into a verb?) with proper punctuation and all, I do let go of capitalization and such when I'm chatting with friends. Still, I think someone could look at my chat logs and at least deduce that I'm a decent writer, just from the way I phrase things—writing is about putting ideas in a readable form, not about being a grammar snob. (A guy once figured out that I'd been an English major, just from listening to my voice mail greeting—I'd lost my voice and therefore had never actually spoken to him—so maybe there's some other signal I'm obliviously putting out.)
I'm just too much in the habit of trying to pick out the single perfect word that will concisely convey my meaning. I've consciously developed a writing voice that's distinct from my speaking one, and I think there's a little bit of ambition evident in even a simple IM, a well-oiled attempt to actually express myself, not simply say somthing.
I think writers will write anytime, anywhere, even if they're not working on the latest opus. I've read blog posts that made me itch to read the author's fiction—or that gave me the opposite reaction. Is that a fair judgment to make? Perhaps not, but it's led me to make some accurate assessments. Not that every word written by a writer will emerge polished and publishable, but I do believe there's a level of craftsmanship that will shine through, the way a master chef will still turn out scrumptious grilled cheese sandwiches.