OMG!
I bought this last night for my iPhone! It has recently been updated for iOS 4 (i.e. higher def graphics and gyro control).
So far it is the best FPS experience I have ever had.
Options - control modes - gyro on.
Sit on a swivel chair with enough room to turn 360degrees and go!
I never felt so much like I was in control of the game. You want to turn around to see what's behind you, then turn around. Each degree of movement made by you is equivalent to one degree of movement in the game. Also, actually aiming at things in order to shoot them is very cool!
Best game for the iPhone I have ever played!
CLICKY