Rockstar formally announced GTA V a few weeks ago, and the other day they released the first trailor

Rockstar Games

The biggest giveaway there is the Vinewood sign, which means at the absolute least this is set in Hollywood but it would obviously be at least Los Angeles/Los Santos. Quite possibly including some surrounding countryside as well.

I think the game looks good, i think it should be able to top GTA4 which IMO went a bit too gritty compared to some of the madness of Vice City and San Andreas.

I was initially hoping they would revisit Miami/Vice City as opposed to going back to California, i wouldve loved to see a map with a large ocean element with all of the islands, bridges, canals, beaches etc for Miami as well as more mainland sections. I think there was a concern though they couldnt go back there without still keeping it set in the 80's, which seemed to be as important as the geography in the setting for GTA:Vice City.