in the US? Yemen is a long way away, and as far as I know the terrorists don't have a competent army or jet planes or missiles or anything that complex. I can't quite picture how that individual in Yemen could pose an "imminent" threat. Can anyone help here?

If the existence of someone in Yemen may cause publicity or news that negatively impacts anyone in the government or corporations protected by the government, they are imminently threatened by that person. If the news DOES make anyone in government or business look bad, the threat has been carried out.

Does that satire answer your question?

but that threat did not become imminent until they were in the US.

A country with air power might pose an imminent threat to the US, but a terrorist in Yemen does not pose an imminent threat. It is someone in the US or near US assets that poses an imminent threat to us.

Some very important information is missing from this story. I have no idea what it is, but as told, it does not make sense to me.

Any plane that passes through a country in which terrorists are believed to reside should be carefully checked.

... like busting MEGA-Video Web sites.

This is beyond effed up.

The Classic Example is the Pinkerton's attempt to kill Jessie James, by throwing a bomb into his mother's house, wounding her (She lost an arm) AND killing their eight year old brother.

More on the bombing:
http://en.wikipedia.org/wiki/Jesse_James#Pinkertons

The Pinkerton's reputation was so "High" among working class people that during the 1892 Homestead Strike, the strikers on heating that the Pinkertons were on they way, stormed the Steel Mill (The strikers had stayed outside of the Mill till then), went to where the Barge with Pinkerton Agents on it was trying to land, and opened fired on the barge. Some of the workers even took an old Civil War Cannon and tried to hit the barge with it (difficult shot, shooting down hill with a muzzle loaded cannon using powder made and marketed for muzzle loading rifles, thus of varying grade so each shot produced a new shot pattern). Fortunity for the Pinkerton Agents the Cannon missed (And the Strike Leaders were doing all they could to STOP the shooting, including the Cannon shots. This incident was the chief reason most Civil War Cannons were removed from larger cities as monuments at that time period, many would stay as war memorials till WWII in small towns, but NOT in Cities).

I bring up Homestead for such killings tend to bring with it hatred and fear as shown in the Shooting of the Pinkerton Agents in 1892. This took decades for the Pinkerton agency to live down (And an abandonment of Strike braking efforts by the Pinkertons do the the change in the law that you had to be a resident of the state for at least one year before you could become a Law Enforcement Officer, the Pinkertons favorite tactic was to bring in a battalion of thugs and have the local Sheriff deputize them, thus making it "legal" for the thugs to kill anyone, the Pinkertons would then reward anyone who did the shooting with their entire pay and a bonus and a railroad ticket out of town, so the shooter would be long gone by the time anyone could file criminal charges against the Shooter).