Enchantment, 3W (4)
Rare
At the beginning of each opponent's combat, separate all creatures that player controls into two piles. Only creatures in the pile of his or her choice can attack this turn.
Illus. Randy Gallegos

8/1/2008: Creatures which enter the battlefield (or cards which become creatures) after the beginning of combat ability resolves are not placed in either pile. Since they weren't in the chosen pile, they won't be able to attack.
8/1/2008: If you have two of these, then you will separate the creatures into piles twice, and the opponent will choose a pile each time. Only creatures that were in both of the chosen piles can attack.
8/1/2008: The effect is best understood if you read it as "creatures not in the chosen pile can't attack."
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander