I have a theory over what Nolen will do about the Joker (since Ledger died). I think Nolen will have the Joker killed. Odds are the Joker will be killed in prison, most likely by someone he pissed offed (and lets face it, the Joker wasn't out to make friends). And we won't see the deed being done, we may just hear about it. But if we do see it (which is unlikely), we will never see the Joker's face.
btw, when I say Nolen, I am referring to the director of the movie, Christopher Nolen.