iPhone 4 32GB, only vibrates half the time.. Vibration is turned on for both sound and silent mode (mode doesn't actually make a difference to the problem I'm experiencing).. Sometimes the vibrate function works when I get a call/SMS, sometimes it doesn't.. I've noticed that when it doesn't vibrate, if I give the phone a bit of a shake or tap (as if to free the accelerometer from getting stuck when the phone wont auto-rotate) then the vibration starts working again.. Phone works fine apart from that, the sounds etc always work, just the vibration sometimes doesn't work, probably at least half the time now.. Any ideas? hardware or software related?