Uhm, according to Christopher Nolan, it is possible. If they've got a ton of footage we haven't seen yet, they can just stick those scenes in here and there, and work around it with the rest of the live actors.
Not necessarily true. The scenes have already been filmed, therefore his heart and soul would be behind the scenes. They'd just be used in a different movie than intended.
The Joker was a fucking epic role, and if they've got a good way of bringing the character back without using some shitty CGI or whatever, I'm all for it. It's not like I'll be watching the new scenes with The Joker, and be thinking to myself, "Damn. This movie sucks. These are pre-filmed scenes!"
Heath was already dead before The Dark Knight came out, he'd just finished his scenes. It'll be the same case in the next movie. He's still going to be dead, he'd just already filmed the scenes.
Hey, if Tupac could keep releasing records after he was dead(Hell, there are two movies coming out with him in it soon!), why can't Heath be in Batman 3 after his death?! :cool