The role of the Riddler has previously been tackled by Frank Gorshin in the television version of Batman, with Jim Carrey wearing the question-mark covered tights in Batman Forever. Meanwhile, the Penguin was handled on television by the incomparable Burgess Meredith, and a distinctively disgusting Penguin was played by Danny DeVito in Batman Returns.
A source told the Enquirer that "(Producers) are convinced that the role of the Riddler is perfect for Depp. Johnny's a pro. He'll be able to take direction from director Chris Nolan and still make the character his own. And what better Penguin is there than Philip Seymour Hoffman."
While it's hard to argue against either Depp as the Riddler or Hoffman as the Penguin, when one considers the original source, it's probably best to consider this one firmly in the 'rumor' category for now.