I know if you pick the phantom class, and are using a sniper rifle as opposed to a regulator, when you scope in (that seems to be the only way of actually hitting things accurately with the sniper unless the baddie is right up in your face and you get lucky) and hit them in the head, there's a robotic male voice that yells out "HEADSHOT!" Which is always gratifying to hear. It seems like it's a one-hit kill, but most things with the sniper rifle are.
As a little side-note, it's really easy to get headshots on a sniper, since you can just use cloaking, then take your time. Mind you, it does remove your cloak if you take one shot, and there is a cooldown on the cloaking skill. So I wouldn't recommend missing that one shot that you can take.
