Delta and American Express just announced their Pay with Miles program, and it’s actually quite good (assuming you have a Gold or Platinum American Express SkyMiles credit card*). In short: You can pay with flights using miles, with 10,000 miles knocking $100 off the flight’s cost. This isn’t great for business class tickets, but it can come in handy for coach tickets. There are plenty of times where, say a ticket to a European city would cost $500-600, but a reward seat isn’t available (on Delta, this would be most of the time). So you would just use 50,000 miles to “buy” the ticket. Voila, free seat.
This deal seems to make the most sense for tickets to Europe where their value (about $500-$600, at least in the non-summer months) is roughly in line with their miles cost (50-60,000 miles). This would also be good for an Asia flight with no reward seats – you’d pay in the $800/80k miles range, but you get the benefit of availability, which is priceless.
* (I had originally posted this information correctly. This is now corrected)