Just an evil thought:

if I was a terrorist and wanted to disrupt American air travel and cost people and industries money-- I wouldn't go as far as blow up planes, etc.: just send my people with clean backgrounds on flights, with "explosive residue" on their otherwise clean shoes, electronics, etc. They would be eventually cleared, but the resulting slow-downs, delays, and mass cluster-f's would wreck as much havoc on the system as 9-11 did, and with little or no cost. If my people get hassled because of their headgear, appearance or "racial profiling": all the much better, a team of CAIR or ACLU attorneys would be standing by to cause even more trouble...