Michael Caine may have spilled the beans:

“I was with [a Warner Bros.] executive and I said, ‘Are we going to make another one?’ They said yeah. I said, ‘How the hell are we going to top Heath? And he says ‘I’ll tell you how you top Heath — Johnny Depp as The Riddler and Philip Seymour Hoffman as The Penguin.’ I said, ‘S–t, they’ve done it again!’” [Laughs]  (Splash Page)

If true, this is something to be excited about.  Unless Nolan falls back on the cliched action film plot/direction of Batman Begins, this casting rumor would signal more brilliant casting for the franchise.  I can see each of these men in these roles.