Monday Webroot tweeted a UK Register story here. mentioning concern about external hard drives.
Also on Monday, CNN quotes Homeland Security as saying that the requirement could be put into place for domestic flights later, since some terrorists have valid US passports and visas and could try to come into the US and disrupt domestic travel. But this has not happened with other means (like underwear).
The latest information suggests that there is specific concern that the battery component of a cell phone or laptop could house explosives. Therefore, starting the device by plugging in to a power source doesn't prove it's safe. Your batteries must work. You need to think carefully about traveling with older equipment.
Wolf Blitzer asks if a laptop could still work while having an explosive: