Some posts earlier on refer to a possible test run for future terrorist plans..... I find this more doubtfull as after attacks of this nature (though we don't KNOW that yet) security would be stepped up dramatically so theories about planting items in the cabin before hand etc etc would be harder to do... I hope! I was on one of the first flights out of Gatwick to the USA 3years ago, and my flight was delayed almost 2 hours, just because of the extra security-hand searching every item of hand baggage etc...
One other thought... would it not be possible for an aircraft to appear to "explode" in mid air if it was say, descending a wee bit too fast, and not necessarily a bomb going off? I seem to remember watching video reconstructions on the TWA 747 where they believe the wings ripped off and the fuel igniting after the initial inflight seperation, due to the rapid speed the main part of the aircraft subsequently acheived. Possible that in a hijacking and consequent struggle for control, the aircraft could be put into an unrecoverable situation, thus massive speed exceedence...