Finally it did happen! In the morning April 29h, 2013, WhiteKnightTwo went for a flight and this time SpaceShipTwo's rocket motor was fired. The motor did ran 16 seconds, and in that time the rocket achieved 1.2 Mach speed. The maximum hight was about 20 km in this flight. The flight went well and there will be more flights coming!

More information:

http://www.virgingalactic.com/news/item/virgin-galactic-breaks-speed-of-sound-in-first-rocket-powered-flight-of-spaceshiptwo/