Ironically, leaving a silenced phone set to vibrate on a hard surface like a table can actually create more of a disturbance when it rings and bounces around. So Apple has patented a system that uses the iPhone's mic and accelerometer to listen for vibrations and scale back its intensity when it gets too loud.
The system is particularly clever because it uses hardware already built into the iPhone. And in a situation where reducing the vibrator still produces too much ambient noise, it can be switched off completely with alternate alerts triggered such as quieter audible alarms or visual cues like flashing the display. Alternately, you can just turn your phone off like everyone suggested in the first place. [US Patent & Trademark Office via Apple Insider]