US Airways is offering double (redeemable, not elite qualifying) miles through December 15, 2010 when paying by Mastercard. If you use a US Airways Mastercard you actually get a 100% bonus. Only tickets purchased after registering are eligible for the bonus.
The promo code, auto-filled in at the link, is MCQ4.
Hmm… I’m getting a “The promotion you’re trying to access might be invalid or might have expired. Please check the URL you entered and try again.”