There is some really quite good 3d on the DS. Metroid Prime Hunters was pretty good, Mario Kart DS also so I wouldn't be surprised if they went that way. 2.5D is also likely.
Most of you are forgetting that the original GTA WAS 3D (It was only cars, peds and explosions that were 2D sprites).
Also, if they hype up a game at E3 they cannot possibly release a GTA:GBA clone because that would just be SO below par for quality in a Rockstar game.
And listen to all those PSP owners wail - you got two GTAs already keep your hair on!
Metroid:
MarioKart:
Though this one I think is the best:
Tony Hawks