When DC and Warner Bros. first announced that they were making a Joker origin movie, no one was really that excited. The thing that makes the Joker such a great villain is that we don't know anything about him. We don't know why he's crazy, and explaining villains just makes them less interesting. The comics have even gone out of their way to present several different origins for the character, some more plausible than others, but fact that we don't know makes it more creepy.
No one seems to have given DC or Warner Bros. that memo, though, and the newest rumor says they are charging ahead with this movie. According to Jeff Sneider, the editor-in-chief of The Tracking Board, the movie is set to start filming this May.
Sneider says he got this information back in January, but decided not to run it. At the moment there isn't any definitive casting in place but there are a few rumors going around, including Joaquin Phoenix — which doesn't make much sense if this is supposed to be an origin movie, as Phoenix is 43 years old. We'll have to see where this one ends up going.