The PS3 will not output game audio via USB, so whether it's a headset or just normal headphones that connect via USB you won't be getting any game audio through them.
I was recently looking into this, and there is a headset that connects both via USB and the digital sound thingy, so you get the game audio, chat and can use the mic. I'll see if i can find the link again, though it was quite expensive.
~ PSN - Trent_Steele ~ Steam
~ Battle.net - Chief#6843