Over at Dark Horizons they are talking about who's signed up for the sequels already. However, this news is coming straight from the stars and it does go to show that they are keen on the sequels.

Christian Bale says that he is definitely signed to play the Dark Knight again. "We got a sense of who he is, but there's so much [more]...I haven't spoken with anybody about this, but I just really think it would be nice to come up with some kind of R-rated version, too, like a movie that is at one time PG and R, where you do two different edits for it."

Christopher Nolan - "I don't really know. I did enjoy making this one. It's a fascinating character."

Morgan Freeman - "I'm not. I'm not sure that they really think that Lucius is an integral part of the series necessarily..."

Gary Oldman - "I'm signed."

Michael Caine & Katie Holmes - Caine said of both of them "No, no. We're not signed." He then turned to Holmes and said "I mean, we'd do it, wouldn't we?" Holmes nodded affirmatively.

Source: themovieblog & dark horizons