I just totall stripped down one of my 6230i's and on the bottom inside there is a tiny motor with an off centre weight on it. This is what makes the phone vibrate, but as to why not whilst charging.....
__________________ It was like that when i got here!!!
when I first noticed this the only reason I could actually come up with is if you ever notice that a phone can tend to vibrate itself off a desk and while this being a problem I can imagine if it's plugged in when it takes the dive and the cord gets taught there's probably going to be a problem with that little connector trying to stop the weight of the phone.
That or the way the circuitry is connected there might be an issue with powering the motor whilst the charging circuitry is enabled.