There are guys on the Beyond3d forums that take raw screens captured via software directly as it is output by the PS3 and then count the pixels in the raw screenshot.
The poster most associated with releasing the raw resolutions of games is Quaz51. He is the member that broke Halo 3's native resolution being 640p.
GW Smokey, the game is then scaled from the raw output to a resolution that is supported by the TV (either 720p or 1080i/p). The game itself isn't natively output at 1080p, but rather, it is scaled up to fit your screen. Couldn't tell the difference, could ya'?
That is why I made the comment about why this all matters. It doesn't. Games in the previous generation of consoles barely ran at 480i in some cases, but we were non the wiser and judged games on other criteria. If you would like to discuss the technical engineering behind the hardware/software, etc, then you should probably head over to Beyond3d, as most of the members there are hardcore programmers/engineers that are
far more qualified to answer questions about this stuff than I am.