On Rockstar's official blog, they have officially announced that Grand Theft Auto V is indeed in full development. No duh, Mr. Holmes.
Rockstar North is taking Grand Theft Auto back to Los Santos and, as suggested in the trailer, encompass the "surrounding hills, countryside and beaches". Most developers like to say that their latest game is "the largest and most ambitious game" they've ever made, but if it's true that they are reimagining "present day Southern Calfornia"... that's a lot of ground to cover.
Though I don't think that the "open-world freedom, storytelling, [and] mission-based gameplay" based "on the pursuit of the almighty dollar" is really all that new in concept for the series, the online multiplayer certainly is. Will it be like Red Dead Redemption's multiplayer or completely, shall I dare, cooperative? That would almost be too shmexy.