The mobile phone ban in Immigration halls at US airports has been in place for some time. Despite the numerous signs and warning announcements, people still turn their phones on.
It's mildly interesting entertainment while in the lines actually. Settle into a line, wait for the "chirrup" of a phone powering up and try to spot which Immigration Officer hears it first. At Boston, they enjoy coming out of their cubicles and swaggering over a la Wayne..."Aww, turn that phone off Pilgrim..."
Bless. It's the only time they really get to show off their handguns and other vital self-defense equipment they need to protect against the feared long-haul airline passenger.