Anti-terrorism strategies: two in the hold are worth one in the overhead compartment

It's been all over the British media today that the Home Office has shifted the security alert to its highest level - a terrorist attack is imminent.

The nature of the attack is said to be a bomb carried onto an aeroplane in someone's hand luggage. There is no indication in the media that the proposed bomb is a dirty bomb, but just a bomb that goes bang and destroys things.

The recommended course of action is for airlines to ban hand luggage until further notice - meaning that people will not be allowed to carry any more than a lipstick and a piece of fruit on board, in a clear plastic bag, like a shopping bag - which seems like overkill given that they are also going to be swabbing passengers for explosive residues. Surely you could hand-search a handbag sized piece of luggage in the time it takes for the swab to be processed, and it would distract the passengers from thinking about how much they are being delayed.

Those questions aside. My big question is, why is a bomb unacceptable in the cabin, but acceptable in checked luggage?

And of course, if it's a dirty bomb, an aeroplane is about the best place for it. Everybody already has an oxygen mask, and you can remove the toxins in about a second by de-pressurising the cabin. If the dirty bomb is in the hold - which isn't usually pressurised - surely there will be bomb residue all over the luggage, which may end-up being tracked all through a foreign city by a bunch of travellers grumbling about all the dust on their luggage after being in the plane.