Here's what I know so far: there will be a huge bank robbery scene led by the Joker and his clown-masked henchmen. This scene WAS filmed in IMAX, which stirred some discussion. It looks like this is the scene we'll be seeing in this prologue. Whether or not this is a scene from the movie or a special "prelude" shot specifically for this IMAX release I'm not sure, but I'm leaning towards the latter.
Either way, this will certainly be the premiere of Heath Ledger's performance as the Joker. I know there's a lot of Ledger-bashing and skepticism on this particular forum, but I'm convinced from what I've read and heard that Ledger's going to surprise EVERYone...and his performance may even exceed Nicholson's. And I'm ready to debate with anyone on this!
(And please, everyone who's reading, I'm going to kindly ask again to spare me the "Brokeback" jokes).