I don't think Lucasarts is stupid enough to leave KOTOR (and Battlefront for that matter) behind. There will be a third. I think Bioware will make it, using the engine created for Mass Effect.
I can't wait for the Force Unleashed to finally see release; not only to play it, but also to see whats next. It is in heavy focus, and we won't know anything about KOTOR III until that has passed. Hell it might even be already in development.