Until end of turn, creatures you control gain "Whenever this creature deals damage to an opponent, you may return target creature that player controls to its owner's hand."
6/1/2011 Only creatures you control on the battlefield when Arm with Aether resolves gain the ability. Creatures you gain control of later in the turn won’t be affected.
6/1/2011 The affected creatures keep the ability even if another player gains control of them that turn. If such a creature deals damage to you, the ability will trigger, targeting a creature you control.
6/1/2011 The granted ability triggers on all damage dealt by the creature to one of your opponents, not just combat damage.
6/1/2011 In a multiplayer game, if an affected creature deals damage to two of your opponents simultaneously, the ability will trigger twice.