These thoughts are based on my current, incomplete knowledge about the process of launching and running an airline. As such, factual inaccuracies might be present.
In mid-January this year, I read the headline of a story on Reuters, which gave me this idea. Is it possible for people-with-malicious-intent to legally launch a civilian airline, and someday use that airline's aircraft for malicious purposes (similar to 9/11, perhaps)?
I tweeted about this, but based on my sister's request, I deleted the tweet (but saved a screenshot). Here's a sanitized version of the screenshot:
Update (20-Feb-10): A news story in The Washington Post (annotated PDF here) and in The New York Times prompted me think - is it even required to launch an airline to conduct destructive acts? What prevents people-with-malicious-intent from directly purchasing low-cost, small-or-medium-sized aircraft from Bombardier/ Cessna/ Dassault/ Embraer/ Gulfstream/ etc., purportedly for business/personal use, and using them for destructive purposes? And this doesn't yet include purchase of second-hand aircraft...
Update 2 (20-Feb-10): A related article on a stolen Boeing on BBC (annotated PDF here)