GamerSushi has dug up an awesome Youtube video showing Fallout 3's V.A.T.S. system in Halo 3.
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The V.A.T.S. system in Fallout 3 allows you to 'pause' the game and target any area of your enemy's body - the head, arms, legs or even weapon.
The video uses some nifty editing to show what Halo 3 would be like if it used the same system. Fortunately, there's no wasting-a-million-rounds-on-Mirelurks action. Maybe that would be a little too close to Fallout 3 action...