US Airways Dividend Miles is offering up to 200 percent bonus miles to new members who sign up for the program through Oct. 15, 2009. The bonus miles will only be offered on all mileage-earning partner activity completed during your first 30 days of membership and there’s a twist–there’s no way to know how many bonus miles you’ll receive. Dividend Miles will randomly select a bonus offer between 25-200 percent and the likelihood of receiving an offer at the lower end is higher. The odds of receiving a 25 percent offer is 80 percent, 50 percent offer is 15 percent, 100 percent offer is four percent and only one percent will receive the 200 percent bonus offer. And not every activity will qualify for the bonus–credit card sign up bonuses, hotel point transfers, mileage transfers and all mileage purchases will not qualify.

Bottom line: New members will probably only earn a 25 percent bonus on mileage-earning activity completed in the first month of membership, which isn’t a great offer, but programs generally don’t offer many incentives for signing up and it won’t cost you anything to become a member. For the lucky one percent, signing up could mean earning triple miles on partner activity for 30 days.