According to FirstShowing.net, they have talked to a source that confirms that The Riddler will indeed be the main nemesis for Batman in Batman 3. Additionally, the source confirmed that Joseph Gordon-Levitt is penciled in with the status of "interested."
While the article goes into great detail that this is only a studio grid and that JGL's listing shouldn't be taken as gospel, I tend to think that it makes perfect sense given director Christopher Nolan's tendencies. He's a director that is fiercely loyal to his actors and likes to cast them in all of his projects. Just ask Michael Caine, Cillian Murphy, Christian Bale, etc. More on this in a day or two.
So, the fact that someone that Nolan worked with on Inception turning up on an official casting grid lends just a bit of credibility in my opinion. And you all know how fanatical I am about anything to do with Christopher Nolan.
It's funny, over two years ago when The Dark Knight came out, there were rumblings about Johnny Depp being set to play the quizzical maniac. After that, you chimed in and gave us some great ideas on who should play The Riddler and a host of other villains in the Batman universe.
No one brought up Joseph Gordon-Levitt, but I think it's safe to assume that he wasn't on anyone's radar for this type of role. Now, after Inception, I'm getting a real Heath Ledger-esque home run vibe from this news.