I can understand if there is a real danger of the radio signals setting off explosives, like at a construction site, but that is a special case and is not common. The myth of the cell phone's ability to ignite gasoline fumes has been disproved enough times that the warnings have started disappearing from gas pumps- at least around here.
But, there are still places of business that forbid the employees from possessing cell phones while working. Not just forbid them from having the phones turned on or using them while "on the clock", but forbidding them from having it in their possession at all. Ridiculous.
This is just as stupid (and just as dangerous) as a prohibition on any other thing that stays totally concealed and doesn't harm anyone's property in any way. And I am just as opposed to this prohibition as any other.