You can't play FPS with a light gun because you need to be able to direct your character, light guns don't offer that functionality. Light guns can only be used in games specifically designed around them currently.
They can play those games because they have PlayStation move. I've got Move on my PS3 and it's pretty good.
I can't see Xbox doing this though as the Marketing line for the rival Kinect system is "You are the controller", if MS bought out a gun accessory for Kinect, that key selling point would no longer apply.