I'm pretty sure it's before. We can only speculate but I'm sure he escapes from the cops when the station is blowing up. The way I imagine it panning out goes-Mr.J is arrested, hauled into an interrogation cell and kept over night. throughout the night we get the mayor asking about him, Rachel having a crack at breaking him, Dent and Gordon observing him and finally Bats storming in to get things sorted (although he fails to get any info out of him). Next morning the Joker is put into the cell we see in the trailers, so the press can finally get pictures of him and stuff. then a series of minor bombs begins destorying the station in stages(most likely sychronised to the Jokers clapping motions), in the commotion he gets out of his cell (maybe stops by to kill the commisioner? ala the beginning of Knightfall, after Arkham blows and rigs up the gun by Dr. Arkhams head), and then him and his goons speed away in a stolen cop car, where you get the truimphant shot of him hanging out the side window. After that I'd assume the film would calm down for a few minutes and the next time we see Mr.J is either broadcasting his threat on TV, or dressed as a cop at Loebs funeral.
I hope they edit it out otherwise it's slopping editing.
