Zoe's example is no exageration. I've seen posts like that here. Not being a good speller doesn't mean you're stupid, but it does get in the way of getting the message accross. I know we have people here that english is a second, third or fourth language for them, and I don't feel they're stupid, often they try harder (and are easier to read) then the worst language manglers on the site.
Use a spell checker, you can get one for free that intergrates with Internet Explorer at http://www.iespell.com/
i don't know how many times that link has been posted.
My spelling and grammer isn't perfect, but at least I put in some effort. If it's not worth doing right it's not worth doing at all.