Google-baiting is the worst. Yes, an actor who previously played a character named Joker will appear in The Dark Knight Rises, but it isn't Heath Ledger, Jack Nicholson, Mark Hamill, or anyone else like that. It's Matthew Modine, who played Joker in Full Metal Jacket. SEO is the worst.
Anyway, Modine will play a character named Nixon and that's all we know. Thanks Christopher Nolan, always willing to give out a wealth of information. Of course, it would be most interesting if it turned out to be Richard Nixon but yeah, that won't happen. For starters, Modine looks nothing like Richard Nixon and second, Batman isn't exactly a superhero who goes traveling through time a lot. Oh well, that's why we have fan fiction.
Also joining the cast is Tom Conti and Joey King. Conti is a well respected British actor, but will probably be best remembered for having an open marriage and maybe having an affair with Princess Diana. There are worse things to be remembered for. King, meanwhile, makes most college graduates look like complete slackers (which we are). She starred in Ramona and Beezus, voiced characters in Horton Hears A Who and Ice Age 3 and also had a role in Battle: Los Angeles. Damn tween, making me look bad.