From what I can figure it's more the risk of a spark generated if you were to drop the phone rather than radiated energy.
Cell phones (mobiles) are fairly flimsy normally and dropping one from waist height normally has the battery bouncing about on the floor.
It's a miniscule risk but it's the old adage of 'never say never'.
There's a ban on using them at petrol stations over here for much the same reasons....
Personally I notice people get more distracted jibbering into their cell phone than they do using a radio - wierd ain't it!