@cwatters: I had to get my us M1 visa in March of this year and I had to leave my mobile out. I left it in a locker at Waterloo. The girls standing checking everyone were already tired of saying "no mobiles" and this was by 10am.
Strange. I've since checked with my wife and her recollection is same as mine. We left at least a two phones, car keys with electronic fobs and our twins Nintendo DS at the security gate. We were renewing US passports.