I found this in the PS3 Game of the Year Awards.
"Best PS3 game of the year? How good is that?! I've always said that PTOM has the smartest, best looking, and most cleverest readers and journos in the entire industry and now you've proved it. As a reward for your impeccable taste, here's confirmation of one of the big secrets pf Batman: Arkham City. If you haven't finished the game then stop reading now.
So, did you think The Joker was actually going to die in the end? Or is he, in fact, even dead? There's been quite a bit of speculation that the Joker somehow survived. Was there an off-camera switcheroo just before Batman walked out of the Monarch Theatre? Was Batman in fact carrying out Clayface(impersonating The Joker)?
So I thought I'd take this opportunity to clear up any confusion once and for all: I can confirm that everyone's favorite enigmatic, charismatic, psychopathic, sociopathic killer is indeed dead. Dead. Dead. Dead. Atleast he was when Batman last checked. Thanks again for the incredible award, and much love to all your readers!"
TLDR: Joker is dead and he is not coming back.