I was in a group where the DM occasionally tag-teamed with one of the players and let him run an adventure. It was infrequent and the adventures were always on the side, with no major story elements revealed or effected. The two collaborated closely for these one-offs and the adventures were always planned ahead of time and DM-approved in advance.
However, personally, as a DM, I have never relinquished the DM-chair for one of my campaigns to anyone else. My feeling is that if someone in my group wants to run a game, I'm more than willing to play, but they need to have their own story to tell.