I was going through the Widgets and noticed a bunch of settings if you select the power control widget. It has exactly the feature I've been asking for and looking for for months now. That is, enable/disable Bluetooth when ext power on/off.
I have a Droid dock in my car and this allows me to not have to toggle bt on and off now when getting in my car.
There's some more controls in there as well, check it out if you haven't already.
Sorry, if you already know this. I'm just excited to have found this!
Sent from OG Droid sporting SS5.4 w/HoneyGingerComb and Chevy's lv1.0ghz kernal