Here is a little how-to guide on how to run Grand Theft Auto at 1080p on the PlayStation 3. If you have a 1080p set and the game keeps booting up in 720p all you need to do is disable the other resolutions, I will show you how to do that now...
Go to to the display option under settings
Select video output settings
Untick 1080i and 720p
Press X again
Please note that you need a 1080p HDTV in the 40" range and preferably connected via HDMI.
Great! Shame on me, i didn't even notice before, that the game was running in 720p only... My Samsung 40" doesn't show the resolution until i press the info button. But this workaround is very good, running 1080p now.