Re: The Joker's background?
Everyone has an origin. I remember that thing about him simply materializing on the sidewalk, but I see this as more of a metaphore rather than a literal statement.
The Nolanverse Joker has an origin, he is the way he is because of something. Now wether he works better as a character if the said backstory is not directly involved is a whole different matter of discussion.
I like it that way - it's as if those movies are from the point of views of Bruce and, to a lesser extent, Gordon, Rachel and Dent, and suddenly - out of nothing - this guy appears and starts raising hell. Nobody knows him, nobody knows what happened to him - he just shows up and begins doing what he does. That's pretty much how it works in real life too - people have their agendas and their past, and when your path intersects with that of another person at some point, it's like they have materialized in your life. But that doesn't mean they just popped into existance two minutes before that.