Well guys, I have to say, having finally taken delivery of my PS3, that I'm loving GT5, the graphics are truly awsome (it detected my TV at 1080p, so that's what I'm using, I thought the TV was only 720p).
If its only 720p its possible your tv will accept 1080p signals anyway, i would test both setting (720 and 1080) and decide which you think looks best because its possible that the 1080 signal is getting squezed into your defenition whereas native resolution is always best....