As an alternative to the interesting eBay deal with Priceline, you can get 3% back on any Priceline purchase through UPromise.
Although UPromise is geared toward (and marketed as) college savings, it's really just a rebate program. Although they don't say it on the front page of the website, looking through their FAQs reveals that you can get them to cut a check for your balance with them at any time.
The calculation of whether 3% cash or 1500 miles is a better deal is an excercise for the reader (but it probably depends on how much you're spending...)