Even with the upgraded engines, the KC-135 (and other variants based on the airframe) are getting pretty old and hard to support. The 767 tanker version has been in the works for some time. There was some controversary earlier this year in regards to whether it's better for the Air Force to lease or buy the airframes. The last I heard, they were going with the lease even though it cost more because it was a sweatheart deal for Boeing. Some say that was largely due to Boeing's lobbyist being married to the Senate Majority Leader.

According to this article, they're also looking at using the 767 airframe to eventually replace the Rivet Joint and JSTARS aircraft. There's also the 767 AWACS, two of which were sold to Japan in the 1990s with more under contract.