If so, SpellBound might be the application for you.

"SpellBound is a port of the spellchecker code and user interface from the Mozilla Suite's Composer that enables spell checking in web forms such as html textarea / input elements (html input password elements are not checked by SpellBound) and rich text form elements. This allows you to spell check forms (e.g. message board posts, blog entries, wysiwyg, etc.) before submitting them when using your Mozilla Firefox or Mozilla Suite browser."

It's a pretty nifty application - I'm using it right now with the Rich Text Editor in Windows FireFox.  Just right click and you're good to go.

As far as putting in an actual spell check component inside the Rich Text Editor?  Hmmm... we'll see.  I'll have to talk with the folks at Yahoo! Mail to see how they implement it, but the developers and engineers are notoriously busy.  Something about the product being #1 in brand or something.