MTG Salvation

5/1/2008 If the permanent is already untapped, you can’t activate its ability. That’s because you can’t pay the “Untap this permanent” cost.

5/1/2008 If a creature with an ability hasn’t been under your control since your most recent turn began, you can’t activate that ability, unless the creature has haste.

5/1/2008 When you activate an ability, you untap the creature with that ability as a cost. The untap can’t be responded to. (The actual ability can be responded to, of course.)