So, how about sending a text message to every phone every 5 minutes. Vendors could agree on a message they will ignore that will just set off the vibration motor in the phone. You'd have to hack the firmware to get around that, which I would consider beyond the skills of most bomb makers, if not all of them.<p>Of course it would take a few years to phase out all the old phones but you could then pretty much spot a potential bomber by checking the age of the IMEI.