There's not a lot I can add to this news that isn't in the article I sourced it from, so I'll just come out and say it:
Sean Penn is rumored to be the one Nolan is pursuing to play the Joker in the next Batman.
AICN makes good points about this interesting bit of casting. The first is that Penn was introduced as the penultimate joker in Fast Times at Ridgemont High. The second is obviously that he has since lost his sense of humor so utterly that some believe he may function as a "black hole," actually drawing humor around him into his dark dense center where it collapses on itself.
The best example took place during the Oscars, at which host Chris Rock asked, "Why is Jude Law in every movie I go to see? He's playing gay, he's playing straight, he's British, he's American...!" To which Penn replied later, "Forgive my compromised sense of humour but I want to answer our host's question about who Jude Law is - he's one of our finest actors.."
It's a more interesting choice than more obvious choices like Mark Hamill or Crispin Glover, anyway. It's interesting the same way that Jack Nicholson must have been interesting, had I known who he was back when the first Batman was released.