A LinkedIn CV of an ex-employee of Rockstar Games said that Rockstar San Diego may be working on GTA 5 together with Rockstar North.
The profile of Jonathan G mentioned that he worked on “Grand Theft Auto 5 at Rockstar San Diego”. Usually Rockstar North makes the Grand Theft Auto games while the studio in San Diego worked on Red Dead Redemption or on the Midnight Club series.
Rockstar North has been the confirmed developer for the open world game, there's no confirmation that both developers are working on GTA 5. The two studios worked together on Max Payne , so it is possible that there's a collaboration.
■ Jensen6