I still believe it will be later this year. Rockstar has probably been working on GTA V ever since GTA IV came out. It just makes such a great year for Take-Two if they release Max Payne 3 and GTA V in 2012. The only problem I could see would be a delay, but the game is probably already finished.
And I also wouldn't be surprised if it releases 12 -21-12.
For all we know, Agent might be coming out next year, and who knows what the other studios are cooking up?