Mythbusters did cover this. Here's a brief summary: link
Bullets fired into the air maintain their lethal capability when they eventually fall back down.
BUSTED / PLAUSIBLE / CONFIRMED <-- yes all 3!!
In the case of a bullet fired at a precisely vertical angle (something extremely difficult for a human being to duplicate), the bullet would tumble, lose its spin, and fall at a much slower speed due to terminal velocity and is therefore rendered less than lethal on impact. However, if a bullet is fired upward at a non-vertical angle (a far more probable possibility), it will maintain its spin and will reach a high enough speed to be lethal on impact. Because of this potentiality, firing a gun into the air is illegal in most states, and even in the states that it is legal, it is not recommended by the police. Also the MythBusters were able to identify two people who had been injured by falling bullets, one of them fatally injured.
(This is the only myth to receive all three ratings at the same time.)