This is Scaled Composites' Proteus aircraft, with Chuck Coleman at the helm, carrying tSpace's 23 percent-scale spaceship mock-up. Proteus is a fully reusable expandable flexible mission aircraft, the center fuselage section is replaced with different modules depending on the mission.
Varifrank has some great pictures and info on Scaled Composites and Transformational Space Corp. doing some really cool stuff for NASA. Varifrank's post is here. There's also more pictures and information at wired.