While it sounds like it would be great for us, a spell checker would no doubt hammer the forum DB server. I doubt a forums server-side spell checker will ever be happening.
If you want to use a spell checker, it?s a simple matter to type up your posts in any word processor with a spell checker, then hit ctrl-A (select all), ctrl-x (cut), switch to the browser?s AT Forum reply field, and hit ctrl-V (paste).