Does #AJAX work on the body field?
According to the FAPI docs textareas do support #ajax, yes.

I'd imagine the problem is because you'll be using a WYSIWYG editor on that field. Such an editor will replace the textarea with an iFrame, and the AJAX events aren't bound to that iFrame (or its inner textarea), so won't get fired when you change the text.

If that is the case, I don't think there's any easy way to overcome the problem except to turn of the WYSIWYG editor for that field.