It is important to note that the X-43 didn't fail, the scramjet
principle didn't fail. It never got that far. The B-52 released the Pegasus
rocket, which was intended to carry the X-43 up to a burst speed at which the X-43 would be launched and its scramjet engines could cut in and take it to Mach 10.
What happened yesterday (2 June 2001) is that the Pegasus booster rocket went wrong. At time of writing it's not known how, but F-18 jets tracking it reported that it veered off course within seconds. Parts were breaking off it. NASA detonated on-board explosives to destroy it at about 7 km above the Pacific, seconds after it left the B-52.
The X-43 itself was never released. This is in no way a setback for the intrinsic technology, only for the support methods. NASA are disappointed, but the recording equipment on board the Pegasus should allow them to track why it underwent catastrophic failure.
On 27 March 2004 the X-43A was successfully launched from a height of 12 000 m, detached from its rocket at 30 000 m, and attained a self-propelled speed of Mach 7 in a flight lasting ten seconds. It then glided and manoeuvred over the Pacific Ocean before splashing down. This speed attained is twice that of the previous fastest (except rockets), the SR-71 Blackbird.