I can’t begin to stress this enough: this is not verified. But right now, all signs point to Joseph Gordon-Levitt (who just kicked ass in Batman 3 director Christopher Nolan’s Inception) being at the top of a fairly short list to play The Riddler.
It makes sense. Nolan tends to re-use actors from film to film, and Gordon-Levitt did a very good job in Inception.
Fun fact, though: Spider-Man alum James Franco was the original choice to play Gordon-Levitt’s role in Inception. Makes one wonder if Franco would be in the same place if he hadn’t passed.
Source is here.