There is a setting to vibrate when picking up your phone if you missed notifications or calls. Settings/advanced features/motions and gestures. Turn off the slider where it says, "alert when phone is picked up". Unless, of course, now that you know what it is, you want the alert.
Check the other settings in motions and gestures to see if anything else got turned on accidentally. Something may be confusing your phone settings, try deleting the phone's cache partition. It's the least disruptive troubleshooting move.
You can also download GoodLock and the Nice Catch module and turn on "vibration history" to see what may be causing the vibration.
