The world may never know if there are or aren't cell-phone bomb jammers out there, but DefenseTech has stumbled on a patent that may shed light on the subject:
The device includes a transmitter that mimics a cell phone base station and a metal horn to concentrate the signal from a 10 milliwatt power source in a single direction. Scanning... a concealed phone... with the tool... tricks it into thinking it is in range of a new network base station and blocks it from any genuine stations in the vicinity.
It seems that the overhead would be a little high to create a full cellphone jammer and, according to folks in Iraq, most IEDs use cordless phones instead of relying on Iraq's spotty cell network.
Phone Bomb Interceptor on the Line? [DefenseTech]