Poor spelling may not be a sign of stupidity, but at this point, so many browsers have a spell check function that it IS a sign of laziness. Like Anyways points out, on a more personal level, it strikes me as being somewhat disrespectful. If you don't care enough about what you're saying to make sure that it's said correctly, why should anyone else care enough about it to make sure it's read correctly?
Or maybe I'm just an English major grammar nazi. Regardless, I like all the good spelling and grammar on this board.