Joker has an origin if you're Alan Moore. And I hope at least in this movie or movie 3 it's explained how he came to be.
Also, I think it's good Nolan is making it almost 2 and half hours. He's cramming in alot of stuff and a movie like this requires enough time and patience and scenes. Spider-Man 3 was all about interweaving and intertwining multiple stories together but Raimi had a hard time letting the movie breathe with just slowing things down instead of gluing this scene with that scene.
I trust Nolan will handle this movie well. The synopsis of the first 6-minutes has gotten me fired up and ready for this. Even the new aintitcoolnews.com pics has made me a believer.
I still however hope Joker is given an origin. I'd love to see Batman and Joker's final fight at a chemical factory and then Joker's makeup becomes permanently stuck on his face, but we'll see if that's the case. If not than I'd love to see by movie's end or movie 3 Harley come in and asked Joker about his life and he toils with her on his life, how he came to be, etc. then that'd be like "A little from column A, a little column B" or "multiple choice."