GT5 Prologue runs at 1280x1080. I don't recall seeing anything about the resolution that the full game will boast. Unless I missed the interview where Yamauchi actually confirms the resolution the game will have...
Just because Prologue used a given resolution during racing segments doesn't mean [at all] that GT5 will go the same exact route.
But even if it does, it still looks better than anything from the competition so I'm more than fine with it.