Just to raise a counter-point, I would contend that the UK liquid terror plot was successful in the terrorist's scorebooks. I bet the operation achieved what the upper level terrorists expected (realistically). The "mules" thought they would make explosions in the sky and I'm sure the upper level wished for that, but overall it achieved its mission:
A good point. The obvious alternative to terrorists being stupid and/or scarce is that their motives are quite different from what's usually assumed.
That being said, I'm also not sure I buy the 'terrorists hate freedom, so they want more big-brother security' theory. That seems much more likely to be a combination of a frightened populace and opportunistic government.
1) They got tremendous publicity.
2) They substantially disrupted air travel. Heathrow was shut down briefly. The cost to industry (all industry) was substantial. http://en.wikipedia.org/wiki/2006_transatlantic_aircraft_plo...
3) The long term effects (carry-on restrictions and other added security hassles) lingered for a long time (even now).
By that criteria, the shoebomber and underwearbomber were fairly successful as well, albeit not nearly as successful as the UK liquid plot.