Text (AL+errata): Untap any number of target creatures. Prevent all combat damage that would be dealt to and dealt by those creatures this turn. [Oracle 2006/10/01]
Can be used on untapped creatures as well. They won't deal damage either. [D'Angelo 1996/07/10] It says to untap any number of target creatures and the other part of the effect works even if the card is already untapped.
Can be used outside of an attack and on any player's creatures. [D'Angelo 1996/11/11]
