NBC News on the decision of the TSA to not allow electronic devices on to planes unless they are charged up:
A U.S. source familiar with the matter said laptop computers are among the devices security screeners may require passengers to turn on. U.S. officials are concerned that a cellphone, tablet, laptop or other electronic device could be used as a bomb.
Some people have questioned why this measure is necessary, given that a potential terrorist could simply pack a device with explosives while retaining the ability to turn it on, but I think they’re missing the point. My guess – and it is a guess – is that someone has worked out how to create an explosive mixture which, when passed through a scanner, looks the same on the screen as a battery. This means you could replace the battery with explosive, but putting it elsewhere would still stand out as abnormal on screen.
Hence the threat: it’s not that someone can pack a device with explosives (something they’ve always been able to do), it’s that they can now do it undetected.