I personally don't care whether or not posters want to spell or type correctly...BUT it does make an impression on me regarding how I view that poster...only because if they don't care how their posts look, then that says something about them...which could be good or bad.
Personally, accuracy is pretty important in both my profession AND my personal life...I am constantly on my wife's case for incorrect English usage...mainly because I don't want her business colleagues to think less of her. I think that if you want to communicate effectively with someone that you need to speak the proper language. But sometimes slang is the better way to go...so sometimes you have to consider the target audience. However, in the context of a message board, there are so many diverse posters that the default should always be correct English usage and spelling...imagine if people started speaking Spanish, French and Italian on the board (whatever happened to the translator, Webby?)...we wouldn't be able to communicate our thoughts to each other effectively.
In short, if you want someone to understand what you are saying and you want your opinions to be heard, then you will make sure that you put it out there in the most understandable format available...and here in the US that format is proper English!
Just a thought...but to each his own!