Below is some info how you can try to get a free round trip ticket by getting two WorldPerks Visa Cards from US bank .
Note: You can exchange 25,000 sign-up miles (15,000 for Signature card + 10,000 for Platinum Card) for a air ticket within US or between Canada and US. See more info here . The air-ticket is not actually free. You have to pay $90 annual fee for WorldPerks® Visa Signature
and $45 annual fee for the Platinum Card. So it makes sense to do this deal only if you plan to buy an expensive ticket.
Bonus terms from US bank website (as of 1/26/07):
One-Time Enrollment Bonus Miles will be awarded when you are approved for a Card and pay your annual fee or use your Card for purchases, whichever comes first. You are eligible to receive one-time Enrollment Bonus Miles for each new WorldPerks Visa Platinum or Visa Signature Card account type. One-time Enrollment Bonus Miles do not apply towards account upgrades. Existing and previous WorldPerks Visa Platinum and Visa Signature Card accounts do not qualify. Please allow 6-8 weeks for your Enrollment Bonus Miles to be credited to your WorldPerks account. 15,000 Enrollment Bonus Miles will be awarded for a new WorldPerks Visa Signature Card and 10,000 Enrollment Bonus Miles will be awarded for a new WorldPerks Visa Platinum Card. Applicants for a WorldPerks Visa Signature Card who only qualify for and receive a WorldPerks Visa Platinum Card earn a 10,000-mile Enrollment Bonus
Link to 15,000 miles offer for WorldPerks Visa Signature card and 10,000 miles offer for WorldPerks Visa Platinum card
Yet another link to bonus info