You can now book one-way trips using AAdvantage miles (that name always bothered me, like "Hilton HHonors").
USA Today says "It's hard to know how many people would use miles for a one-way trip — parents driving a child to college and flying home might — and American officials didn't offer any estimates." (http://www.usatoday.com/travel/flights/2009-05-11-aa-frequent-flier-revamp_N.htm).
Well, I can provide a personal example. I would use the miles to book a transatlantic flight, using half the miles which I would normally use, and book a cheap one-way return trip with Aer Lingus (through Dublin) to get me back. Hey presto, a cheap way to get to Nice, or Rome, or London. With so many airlines now effectively "one way airlines", it makes sense for them all to move to "one way mileage rewards".