If it's something I can gloss over as a typo (including things like its when it should be it's, or the "a" before vowel culprit), and it's not a formal document, I don't mind so much. What does bother me in emails, IM, etc., is the lack of punctuation and capital letters. Or the gratuitous use of "txt-nyms" and dropping vowels without the excuse of being on a phone or using twitter. (And sometimes even then!)
ths wld ttly make me wnt 2 thrw somthing!!!! O.o
Frequency also makes a huge difference. Once or twice, most likely they're typos. Littered throughout, then someone doesn't know what he's doing, or doesn't care.