Kotaku is reporting that a source from Rockstar Games told them that Grand Theft Auto V will be set in Los Angeles, or a facsimile of L.A., much like Grand Theft Auto IV was set in an ersatz version of New York City. The game will also allegedly have multiple protagonists.
According to Kotaku, "GTA V will be set in some version of L.A., bringing the famous franchise to a place that last got the GTA treatment in the series' top-selling Grand Theft Auto: San Andreas... We're also hearing from multiple sources that the game will feature more than just one playable character."
Rockstar hasn't confirmed or denied Kotaku's claims. A trailer will debut on 2nd November, and undoubtedly answer those questions.