Per Anjow's latest post, could it be that we're going to get a very vague, montage-like origin that is cut together from random scenes?
What I mean is, rather than the narrative flashbacks in "Begins," we get shots of different things - a mask, perhaps, or a shot of chemically-burned hands, or some sort of trauma that relates to Joker's psychosis - i.e., a shot of his eyes (and eyes only) just after he's been burned, etc.
This way, there's no definitive origin story, but we get a dramatic, mysterious, and somewhat disturbing sense of insight into who he is.
Also, considering the earlier info - it sounds to me like the Joker starts off as the Joker, white skin and all. And then, at some point, Batman scars him - perhaps it alters his smile, or it literally marks his skin in some way. At that point, he goes uber-freakin' crazy and their near-legendary rivarly is born. In other words, while Batman is not implicit in actually creating the Joker, he is responsible (in some way) for making him the Joker we know and love - Gotham's top criminal.