A few months back when writing my own message board system (see my homenode if you're interested) I ran across the same problem. I eventually found the open source LiveJournal code that includes a module (LJ::SpellCheck) for spellchecking (it's just a handy interface to any ispell/aspell-like spellchecker). It works well for my board, although I did have to modify it a little to produce output in the manner I wanted.

Nota Bene: If you have HTML in your input, you'll probably want to filter that out before using the module.