Well, tomorrow I can try to edit GoW3's PARAM, to make it support 1080p and give it a shot. But right now it it 03:37 at morning, and I'm kinda sleepy...
Edit: Ok, just managed to make a screenshot of GoW3 @ 1920x1080.
First I've checked the PARAM, and it already said 1080p among the other resolutions. The I went to the PS3's "Display Settings" and made a custom video mode for the HDMI in which I removed all other resolutions exept 1080p (576p stays by default) and with this actually forced GoW3 to play in 1080p.
Once again, here's the proof:
Click here to see full text
Again - don't forget to click on the picture for full res.