I think it completely depends on context. I write any old crap on Yahoo Messenger, because I'm talking to friends.
However, when posting to a forum, I firmly believe one of the fastest ways to cast doubt on your own intelligence is to misspell (I'm in trouble now, I don't know that I'm sure how to spell that) common words or use poor grammar. EXACTLY the same as listening to someone who mispronounces words or who has poor grammar. In fact, there is a thread floating around which criticises Bush for exactly that "crime".
Personally, the butchering of the spelling of
ridiculous really pisses me off. There is no "e" in there. Ever.