TinyMCE has the ability to support a spell checker. I realize many browsers use this, but I have still had users request it. I don't think it would take very much to get it working here in XF--basically, just add the spell checker button to the editor, add a helper file (and verify apsell is installed), and handle the pop-up window the spell checker would likely create.
Server based spell checkers are evil. Opera, Firefox, Chrome, Safari, all have native spell checkers. IE lacks one, but has addons (speckie and iespell) and IE10 should have a spell checker. Browser & system spell checkers are so much better than anything we could ever give them. Its really not funny.