IMO people should at least make an effort at good spelling habits, even on message boards.
Spelling errors that happen once and a while don't bother me, but people who make NO ATTEMPT to make thier spelling legible bug the heck out of me.
Why should I care what you have to say if you can't take the time to check your spelling?
The other thing that bugs me too, is people who consistantly mis-spell the same word over and over.
There is a member of a GI Joe board I go to who frequently spells "like" as "leik" It drives me nuts, and though it has been pointed out to him several times he just doesn't care.