Soundtrack.net has an interview with Hans Zimmer, which in part of it he talks about his upcoming work for The Dark Knight, along with James Newton Howard. He's what he has to say:

You mentioned before that you were probably going to be working on The Dark Knight, but it's not certain...

Nothing is ever certain. They haven't even finished the script yet.

Assuming you do it, though, would you be able pull James Newton Howard back in?

Absolutely! I should say we are doing Batman. I wouldn't leave home without him!

In the score to the first film, the main "Batman" theme only shows up towards the end, kind of like a springboard for the second film...

Absolutely, that's the whole idea. It's Batman Begins! There's a whole Theme that's written and on purpose not in the movie. We were basically betting that this movie might work out alright and there would be another one, so we wanted the character to develop. He hasn't earned that theme yet!

OK, Mr. Zimmer. Hope you keep on the good work.