Yeah, it bugs the shit out of me as well! Small amounts of spelling mistakes and grammatical mistakes are easily forgivable, but when the mesage looks like it's been written by Nell of the woods (see the Jodie Foster movie Nell if you don't know what I mean) it totally drives me crazy.
Whatever happened to being proud because you can speak and write well? Now people are proud to be illiterate, and what's worse is other people will jump all over someone who complains, or just simply points it out, to defend the person instead of saying "Yeah I agree, learn to write". Don't get me wrong, there are people who simply can't help it and exceptions can be made, but when it's just plain stupidity and ignorance then they should be yelled at and made to smarten up.