In the newest edition of GameInformer, they have stated that Rockstar may be releasing the next installment of GTA by the end of ’09.
“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
If this rumor is true, this is excellent news! But until a confirmation by Rockstar is made, take this with a grain of salt.