According to Loose Talk in the latest Game Informer, RockStar North is already working on the next instalment in the Grand Theft Auto franchise, which should be ready for release this holiday season.
"We're not talking about new DLC or a collection of previously available DLC, we're talking about a full retail game. We expect this title to follow the suit of previous GTAs and use a subtitle such as GTA: San Andreas, etc. Tokyo might be a possible setting for the game, but details are non-existent at this time." - Game Informer Issue 190
It wouldn't be surprising for RockStar to release the next GTA this soon – as you may recall, Vice City hit the shops only a year after Grand Theft Auto III was released for PlayStation 2.
Stay tuned for more details as they break.