GT5 Prologue works in 1280x1080 and uses PS3 upscaling trick - by repeating some pixels in anamorphic picture to 1920x1080. So 50% upscale.(It's actually less than GT HD's 1440x1080)
1280x1080 and 2xAA
1280x720 and 4xAA
Confirmed by the same guy from Halo 3 resolution discovery (Quaz51).
"GT5 prologue demo is 1280x1080p (yamauchi know than in car game the vertical resolution is the most important for the visibility) but with MSAA 2x now ! it's a very good Compromise
they also very improved the display of grills and electric wires and aliasing problems (compare electric wires in GT-HD and in this demo, it's big big gap), the texture filtering is very clean and the HDR ligthing is awesome! All this in 60fps with 16 photorealistics cars, it's impressive the show room and the pit/garage is full 1920x1080 no AA"