A Rockstar North employee, character animator Alex O'Dwyer, has listed on his LinkedIn resume that he's working on Grand Theft Auto V. There's nothing unusual in that unto itself, since the game was announced by Rockstar months ago. However, his resume lists the game as having an October 2012 release date.
Of course, O'Dwyer may just be speculating on his own, but it's hard to believe he'd use anything but a "TBA" if he didn't have some sort of release window.
Neither O'Dwyer nor Rockstar have commented on the situation.