Have you ever been on a call on your iPhone when an SMS or email or push notification came in only to experience a terrible vibration right in your ear? iPhone has a setting to enable and disable vibrate both for silent mode (ringer switch off) and in general so you can set it up to ring, ring and vibrate, or just vibrate. What you can't seem to do is set it up not to vibrate when you're already on a call.
What's curious is that not everyone seems to complain about this, and some people seem to only notice it after a firmware update. Forum member iErick as well as others have all been experiencing this issue ever since they made the upgrade to iOS 4.2.1.