According to this Aviation Week article from October 2012, the US Air Force Research Laboratory ran a program called Surfing Aircraft Vortices for Energy ($AVE) to test/prove that autopilot settings could be adjusted to allow C-17s to fly in formation using the wingtip vortices of the lead plane to reduce fuel consumption of the following planes. They seem to have found upwards of a 10% reduction in fuel for the following planes.
The article indicated that previous experiments involved hand-flying of the aircraft in formation which required the equivalent concentration, focus and exhaustion levels of aerial refueling and was unfeasible for long duration flights, therefore the tests proposed to incorporate the formation flying into the autopilot
Does anyone know if these auto-pilot configurations have been put into use in line C-17s or other USAF transport aircraft?