Willie Nelson with Stephen Colbert - "Little Dealer Boy" I don't know if that's the real title. That seems to be the most common name for it on the blogs. This was recorded from the Colbert Christmas Special that aired last week.
All the songs on the special are available for legit download from that one store that everyone goes to for legit downloads. A portion of the proceeds are going to charity, too, so go download them. The sound quality will be better, and you won't have the audience laughing over the track.
Better yet, buy the DVD of the special. The visuals make the performance even better.