So, Virgin Galactic is pretty much the only company working on a carrier-launcher system that involves flying your rocket up to a plane's ceiling and then launching it from there. (This system was toyed with by the military back in the 1950s and Pima Air Museum has a B-52 that was converted for the program).
SpaceShip Two is launched from the White Knight carrier, but the maximum payload of White Knight is fairly low, and it's not really designed to put things into orbit, but rather to send paying passengers on the most expensive thrill ride in history. The program has been grounded since last fall's test flight went fatally wrong, but the next version of SpaceShip Two will launch in February.
But Virgin also wants to get into the satellite launch business. And that means upping the payload. Which means a bigger plane.
Which means...well...they don't have any B-52s to convert. What they do have is quite a few aging 747s. (Technically, Virgin Atlantic has them, but when the same person owns both companies...) Virgin Galactic has purchased "Cosmic Girl" (Yes, it already had that name, no that's not why they picked it) from their sister company and are now retrofitting her to launch rockets from a mount point normally used to carry a fifth engine (which is generally done as an economic way of moving spare engines around).
This has to be one of the best ideas in the history of space flight. Cosmic Girl can take off from any runway which can handle 747s. She can then fly to the perfect, optimum position for launch, release the rocket at max cruise height and speed (35,000 feet and 500 mph) before returning to base. What makes this extra brilliant, though, is that maintenance on the planes can be done by normal airport personnel, as can refueling. And, of course, they can head hunt any qualified 747 pilot to handle the flights. A launch specialist will occupy the third seat which used to be used by flight engineers back when commercial airliners still carried them. The cost savings from using an existing airframe are pretty small. The cost savings from not having to qualify specialist pilots? Substantial.
(Yes, I do have a personal connection to Virgin Galactic, but I really do think this is brilliant).