While just this 6th of July, the Transportation Security Administration in the USA banned "powerless devices" from flights, the UK has joined in the battle this week. Attempting to go through security at an airport going into or out of the UK through "certain airports" will result in your devices being checked (as per usual.) If you are asked to power your device on, and you can’t, your device will be confiscated.
Confiscated in this case does not mean that you’ll necessarily lose the device forever. In the UK, this means you’ll have some choices. You can do the following:
• Give an address where the device should be mailed.
• Allow the device to stay at the airport until you return.
• Throw the device in the garbage.
In the USA, things aren’t so cut and dry. According to the original TSA release, "powerless devices will not be permitted onboard the aircraft." That’s it. Nothing about forwarding or collecting of the device in the future.
In all cases, the traveler - you - will likely undergo additional screening measures. The USA release by the TSA does not say whether or not travelers will be able to board flights once their devices are confiscated. The UK release suggests that travelers will still be able to fly, albeit without their devices in tow.
In any case - make sure your devices are powered up and avoid this situation entirely!
BONUS: Expect a lot more of the following outside the security checkpoint, that's for certain.