This has been raised a couple of times here but to my knowledge no-one has explained what certification 'ticket' they are working towards and who will decide when the beast is fit to carry commercial passengers.
SpaceShipTwo will not be a "certified" aircraft. That's because none of the current commercial certification regimes apply to sub-orbital flight (they are designed for "normal" passenger carrying flights), and there is not enough data available to create a new certification standard.
Note: in a certification regime, not only the aircraft must be certified, but also the pilots, airline, mechanics, etc. E.g., the FAA would have to make a new Category / Class and perhaps new Type Ratings for sub-orbital flight, issue new pilot exam "PTS" standards, update A&P IA requirements to work on rocket motors, issue new FAR Part 1XX for sub-orbital ops, etc.
So instead of going through a (non-existent) certification regime, SpaceShipTwo (and other manned sub-orbital flights) will go through a
licensing regime instead.
Under a licensing regime, instead of certifying the aircraft, pilot, mechanic, etc., the FAA will license the
launch operation.
Prior to a launch, the FAA will want to see documentation, test flight data, etc., that provides reasonable assurance that the launch will be successful. The criteria is to assure public safety and occupant safety.
Currently the FAA has issued single launch licenses (permits) -- a new launch license was required for
each of SpaceShipOne's sub-orbital flights, for example.
Once launches become more routine, the FAA can issue a multiple-launch license for the launch operation.
Now that we've had an in-flight fatality, the FAA will expect even more documentation/data before issuing a new launch license.