Well, looks like the next gta game WILL be GTA4 after all.
It has exact dates for both games and here are info on the two games (from gta-sanandreas.com)...

VICE CITY STORIES:

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Rockstar has announced Vice City Stories, a new instalment in the Grand Theft Auto series exclusive to the PSP, which will be available in the UK on October 20th.

The new game is set in the familiar locale of Vice City, but it won't be a rehash of any previous games in the series - nope, Rockstar is promising "an entirely new game with a new storyline, new missions and gameplay."

It's being developed by the same team that created Liberty City Stories, and is set to feature professional voice talent, a "diverse" soundtrack and all the hallmarks of the GTA franchise we've come to expect. According to Rockstar, this one's going to "thrill long-time fans and excite new PSP system gamers like never before."

Due to be out this October it will be exclusively for the PSP and we will let you know more as soon as we do!hr>

Official site: rockstargames.com/vicecitystories/

GTA IV

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Thankfully it shows for PS3 as well as XBox 360, and won't be a one console game and both will be launching at the same time, apparently

Gamespot posted on their site:

[UPDATE] At its pre-E3 press conference, Microsoft confirmed that the next Grand Theft Auto is coming to the Xbox 360 in October of next year. In an official announcement, Rockstar Games confirmed simultaneous launch on PS3.

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Official Site: rockstargames.com/grandtheftauto4/

I would also like to point the Xbox Live sign on the bottom of the GTA4 site, it may just suggest online (for the 360 at least).