2010-06-15 : If, during its controller’s declare attackers step, a creature affected by Rage Nimbus’s activated ability is tapped, is affected by a spell or ability that says it can’t attack, or is affected by “summoning sickness,” then that creature doesn’t attack. If there’s a cost associated with having that creature attack, its controller isn’t forced to pay that cost, so the creature doesn’t have to attack in that case either.