One thing I wonder though is if we'll see GT6 on PS3. I mean, I know GT4 still took 4 years after GT3 on the same console, but there's only so much you can do to improve certain things, especially like graphics. The 500 or so cars we'll get in GT5 will be top notch so they'd definitely be good enough for another PS3 release, and the same goes for tracks (although with some minor updates maybe). So from there you just add more cars, more tracks, improve the physics and add some other features and you have a a new game. Then again, it's hard to be optimistic like that since we're still waiting for a game that was announced before the PS3 even launched...
But for a comparison, look at the PSX. GT1 took like 7 years to make but GT2 only took like 1-2 years. I'm pretty sure it used the exact same driving engine and physics so the only real difference was the extra cars and tracks. I'm sure the team was bigger than the like 7 person staff of GT1 but the foundation was already done.