This plot has been in the works for years. It was called off for Christmas 2003 because Gadahn started spouting off on the plot on the Internet which was called "Jihad Falcon" at the time at the feds shut down a bunch of flight. There was a ground operation planned, too.
http://www.freerepublic.com/focus/f-news/1307764/posts?q=1&&page=1
12/21/03 EOM - Death to Amerikka - you murdering Amerikkans you have only days left until Allah administers his wrath upon you.
12/22/03 EOM - events will begin in London.......
12/31/03 Internet posts suggested attacks, thousands of fatalities and destruction eclipsing that of 9/11. Deathblow to America promised by early February. Targets included NY, Washington DC, LA, and Las Vegas. They have started the countdown - an American city will disappear. Request for Muslim brothers in America to depart cities of Washington, DC, NY City and Los Angeles. Jihad falcon referenced.
holy cr*p. Thanks for the link (I think). My anxiety level is up about five notches today.