There are two separate issues:
1) headshots
These can only be done with the sniperrifle or the ripper. When your opponent dies, UT will check if you killed the opponent with one of these two weapons. If you did, UT will check where the last bullet/blade hit your opponent. If he was hit in the head (upper one third of the collision box), then UT will give you a headshot.
2) dying animation. when a player dies, UT will show a little animation of the person dying. Most models have multiple animations. One animation will show your opponent's head being blown off. When your opponent dies, UT will check where the last bullet/rocket/grenade/blade/etc hit him. If he was hit in the head, UT will play the animation where it looks like your opponent lost his head.
You should realize that 1) and 2) are not strictly connected. You can kill someone by shooting him in the head, but unless it was a sniperrifle or ripper, you will not get an official headshot award.
Did you play Unreal1 ? I loved it when you blew off the head of a Skaarj. He would use both hands to try and feel where is head was. Just when he was about to realize he didn't have a head anymore, he would drop dead on the floor. Lovely.
Oh, one more detail. When you make a headshot with sniper or ripper, if you hit your opponent in the head, he will receive much larger damage then when you hit him in the body (110 healthpoints vs 35 healthpoints or something). With all the other weapons, it doesn't matter where you hit him. Damage is the same regardless of point of impact.