The X-37 was transferred to DARPA last September. The same article that announced it also metioned that the White Knight would be the carrier aircraft. The B-52 used for so many NASA test flights has been retired and the White Knight was a much more economical aircraft for the job.

The White Knight probably isn't big enough to carry some of the payloads that the B-52 used to carry, such as the Pegasus booster with an X-43. They can extend the wingspan to about 98 feet if necessary. They'll also be working on a bigger carrier airplane for SpaceShipTwo.

The Air Force has been studying space plane concepts for some time now and was a partner with NASA on several projects. Since NASA is cutting back on some research, turning the project over to DARPA keeps it alive. It may also support development of the X-41 Common Aero Vehicle.