OK, been sitting and chewing on this one since the Xmas attempted bombing of a Northwest flight.
Call me a bit jaded, but seems to me there is a helluva difference between an isolated "we are going to blow up the plane" sort of event and a "we are going to coordinate an attack by taking over multiple airliners and fly them into national landmarks" attacks. The former have been with us for years; the latter only since the 9/11 attacks. And as much as we want to travel safely, we've never quite figured out how to prevent the former. Sometimes airlines drop out of the sky due to mechanical failure or human error or weather, less often due to terrorism. And sometimes, miraculously and inexplicably, tragedy is averted. (remember that flight where the door blew off, someone got sucked out, but the plane landed? and of course, the recent Hudson River USAir landing)
My point is, that the TSA and Homeland Security schemes are working. At most we've had a few half-hearted and aborted attempts to take down planes (not, please note, to actually hijack and weaponize planes). And although these attempts may be perhaps part of a greater plan to test the security system and screening, I'd argue that we've learned a lot more from these than the terrorists.
In light of a natural disaster in Haiti that has killed perhaps 100's of thousands and impacted millions, I sit back and wonder how much of an impact even a tiny percentage of the money we've spent on homeland security and several wars would have made had it been diverted to improve the economy, health care, infrastructure, and residences of the poorest nation on earth, right at our doorstep.