The reason for the surprise restrictions isn't clear (Homeland Security isn't commenting as of this writing), but this kind of sudden yet short ban tends to be in response to a specific and imminent terrorist threat. The US' Transportation Security Administration is already worried about bogus devices being used to smuggle explosives, weapons and contraband aboard flights -- this suggests that there may have been plans for just such an attack. While there could be alternate motives for the ban, it's the most plausible explanation so far.

Update: A US official speaking anonymously to the BBC says the device ban would affect nine airlines in 10 airports. Also, it is believed to be in response to intelligence reports hinting at threats.

Update 2: Our initial report listed this as a temporary ban, but officials speaking to TechCrunch and the AP characterize this as indefinite -- the airlines received 96 hours to implement the changes. We apologize for the error. Also, the Department of Homeland Security has confirmed the core details of the ban. It covers one major airport each in Abu Dhabi, Dubai, Egypt, Jordan, Kuwait, Morocco and Turkey, as well as two airports in Saudi Arabia. Flights leaving the US aren't affected.