Ed Morrissey provides an excellent summary of what may be an ominous series of terrorist dry-runs. The TSA alert (PDF) suggests that an attack on airliners may be imminent. The TSA discovered items that closely resembled C4 (explosives), IEDs, and remote triggers.
Furthermore, the explanations of the persons involved were "questionable" and investigations continue.
Noteworthy incidents include the following:
5 July 2007, San Diego, CA – A U.S. Person’s (USPER) checked baggage contained two icepacks covered in duct tape. The icepacks had clay inside them rather than the normal blue gel.
4 June 2007, Milwaukee, WI – The carry-on baggage of a USPER contained several items resembling IED components, such as a wire coil wrapped around a possible initiator, an electrical switch, batteries, three tubes, and two blocks of cheese
8 November 2006, Houston, TX – A USPER’s checked baggage contained a plastic bag with a 9-volt battery, wires, a block of brown clay-like minerals, and pipes.
16 September 2006, Baltimore, MD – The checked baggage of a couple contained a plastic bag with a block of processed cheese taped to another plastic bag holding a cellular phone charger.
Maybe some enterprising reporter will actually ask a question about terrorism at the next Democratic debate, preferably before an airliner goes down.