Why is the origin of the Joker even important?
Oh come on. Why so serious?
The answer is: Because depending of interpretation, you can red all kinds of statements from between the lines and base a whole line of education in interpretation of literature, movies and other art forms.
In this case, you can read more or less subtle message about war veterans lives being completely screwed up by post traumatic stress disorders, psychosis and other souvenirs from the frontlines. In other words, do you know how Joker got his scars...?
And the best - or worst depending on interpretation [pun intended] - thing about it is that every interpretation, given sufficient argumentation backing it up, is equally accurate. Perhaps it was his drunk fiend father... perhaps not.
Regarding the movie, saw it finally. Awesomeness. The only thing that irked me was the appearence of dei ex machinae, like the Joker apparently having set up explosives all over Gotham City beforehand, or having an absolutely huge army of ninjas specializing on infiltration and sabotage at his disposal... like how he managed to rig the hospital and the ships. Normal thugs can't do that, they'd be spotted. Joker himself would have taken a lot of time to rig the hospital secretly, leaving him barely any time to do anything else... therefore, ninjas. Or wizards. Let's say ninja wizards to be sure.
Also, why use spoiler tags in a thread about the Dark Knight? Shouldn't the thread topic kinda, you know, imply that the discussion might concern the movie in question and thus have some details of the plot...?
EDIT: Oh snap, just thought of something blatantly obvious...
-The dead know only one thing: it is better to be alive. -Pvt. Joker
