From the 2008 press release above: "Delta Air Lines (NYSE: DAL) in September will launch a new Award Travel structure to provide SkyMiles members greater flexibility and more options when redeeming miles, including the ability to again book the last seat on a flight using miles."

A supervisor confirmed this rule for me when I had a problem, couldn't tell me why a particular flight I wanted wasn't easily available with miles, and manually opened it for me.