The problem with that is give someone so much as half an inch and there will be someone who extends that by a mile. It only takes one person to abuse it before everyone else does it.
I'm amazed at how relaxed where I work are in relation to mobile phones. Mine always in my pocket apart from when I'm on break/lunch I've had it out on my desk next to my work PC because I've been expecting an important call and I will always seek permission to take it if my operations manager doesn't mind. Normally not a problem.
My colleague who sits next to me, is always on his phone on Facebook which I think to be honest really extracts the pith. I'm certainly not going to grass him in but give him enough rope and he will hang himself.
Its far better to just blanket ban it, then people know where they stand rather than some half baked not during works time but fine in your own break time situation, someone will defiantly abuse that privilege if given the chance.