Having personally used Multiple Data Mining techniques for several years now - It's not that Data Mining doesn't work, rather it's how its used. Data Mining is great at trend forecasting and if you're really good at what you're doing in it you can factor in probabilities of certain future events. The one key factor in data mining is a "Training Set" of Data to teach the machine(s) how to recognize the patterns. Since I suspect Terrorist come from every walk of life, every know nationality, and are using 1 off events this is throwing them a few headaches. The real key is to of course define what is normal, but if the rest of the world is as normal as are we here in the US they don't have a chance to pin point the Target Data (in this case people).
I would also suspect that the Terrorist Motives might be a key factor, but it's like pulling teeth to get any US Administration to admit that their foreign policy is screw up beyond belief, let alone something like a cruddy foreign policy might just result in cruddy foreign relations or popular uprisings around the world. If they did, then we wouldn't need data mining in the first place.
"May You - Live Long and Prosper in Interesting Times" -- by deweycheetham