I doubt it'd be a 7 minute clip of the Joker alone, since Nolan's gone on record as saying he'd rather not reveal or show anything until the film's released.
2 options - far more likely:-
(1) The bank robbery scene - It's been shot especially for IMAX screens, so very likely to be the scene in question.
(2) A Godzilla / Campaign scene - e.g. a phone in where you hear the Joker, see some of the things he does, and end with the Joker's face.
Either way, I doubt you'll 'see' more than 10 seconds of the Joker - you'll probably hear him, see him move, etc. but that's it.
Don't expect a New Goblin/Peter Parker scene, a la Spiderman 3, because that's NOT what Nolan/WB are releasing this for. I think it is the Robbery scene, and it's being released to gauge IMAX audiences reactions to mainstream Hollywood movies filmed for IMAX. (Just imagine BB3, Justice League, or even HP7 filmed for IMAX!!!)