At the beginning of combat on each opponent's turn, separate all creatures that player controls into two piles. Only creatures in the pile of their choice can attack this turn.
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.”