Some time ago, Insomniac had stated that Resistance: Fall of Man would be able to support the 1080p resolution. However, today at Sony's Gamer's Day conference, they went back on that comment, now stating that the game will run up to 720p. This is what they said:
"Native 1080p (versus 720p scaled to 1080p) uses much more VRAM than 720p. When we finished up a few of our bigger levels at the very end of the development process we realized that we would have had to steal VRAM from some of our characters and environments to run in native 1080p. Anyway, we felt it was best to stay at 720p and that, at least while 1080p televisions are still relatively scarce, we should focus on implementing 1080p for our future games."
This is unlucky for Sony to have a developer saying they don't have the resources or perhaps a sufficient understanding of the PS3 hardware to implement 1080p at this stage. However, Insomniac seem confident of fulfilling this in the future.
News source: PS3 Land