I disagree. Done right, it can be quite a thing.
In this case, it would show off the one guy we will have waited for a solid year-plus to see, with no long build-up or bullcrap (most folks here are going to be so spoiled on every aspect of the movie anyway by next June, there's no sense dragging it out

).
But you give that jolt, and then work back to it. Gives a "target" to reach for, and you know that all the stuff you're seeing eventually leads to that point you've already been teased to.
I don't know. I could see it working...the whole time you're waiting the "flashback" or build-up, you're drawn-in because you'll want to connect those dots, and you're eager to see all the things that led to that big "reveal" at the bank. You'll be seeing little glimpses - physically, emotionally and mentally - along the way (early crimes, initial encounters and skirmishes with law enforcement and Batman, things deteriorating in Gotham, etc.) and by the time it works its way back to the bank, you've got this nice, satisfying "click" or connection. Then the final 30 minutes or so could just be a balls-out "Batman vs. Joker" showdown, both in full-tilt form, sparring mentally, fighting physically, etc.
Just don't kill the Joker...save him to carry on in part 3. I don't want this Nolan series to follow the previous ones, and feel like they have to introduce every villain in the world, getting sillier with each one.
Joker is the ultimate villain. Let it be in this one, and leave him alive (back in Arkham or uncaptured) to leave a doozy of a plot open for a third (and final) film, without having to fall back on silly penguins or alligator people.
I wouldn't mind a feline woman being hinted at, and slyly introduced, in this second installment, to have a larger role in the third one...if done right.