G.43.1- To "untap" a permanent means to straighten a card's orientation to show that it is now ready to be used. Permanents normally untap during the untap step (see Phase P.5). [CompRules 1999/04/23]
G.43.2- You cannot untap an already untapped permanent as a cost. Costs must be successfully paid. [D'Angelo 1996/12/23]
G.43.3- You can untap an already untapped permanent as part of the effect of a spell or ability. This has no effect on the permanent. [Duelist Magazine #5, Page 23, 1995/05/01]
G.43.Ruling.1- Untapping a permanent with an effect will never counter or otherwise cancel any already announced ability of that permanent. It merely makes the card available for them to use it again. [bethmo 1994/06/01]
G.43.Ruling.2- A card with an ability to untap the card by paying a cost can be untapped by an effect such as Twiddle without paying the cost. [D'Angelo 1999/05/01]