I was just browsing on Ebay and found this:

http://cgi.ebay.com/ws/eBayISAPI.dll...category=19272

It is a certificate good for 500 miles on any one of Continental Airlines , Delta Air Lines , Northwest Airlines and US Airways. But that is still only 500 miles.

At the time I found it, with over a day left in the auction, it was going for $31.00!!!!