This theory was advanced to me by a coworker and I had to admit I'd had it happen to me: feeling vibrations in the area of your body where you carry your cell phone, and the phone wasn't doing anything. Sometimes even when the phone's not there.
For me, it's my left thigh. It seems to be a touch trigger-happy. A friend said it was his left pectoral - he keeps his blackberry in the pocket of his dress shirt. One day he thought he was having a heart attack.
Ask around and I don't think you'll find too many people who haven't experienced it.