Are you ending the Music app before you unplug? or just unplugging?
Might have something to do with it.
It also helps to post questions in the correct forum.
I listen to Slacker and Pandora in the car a lot since radio stations around here are terrible. When I shut down Slacker for example I hit "Quit" and exit the app, and refuse to subscribe to Slacker Plus, take the 3.5mm jack out of the Droid and in the house I go. I'll look down at the phone a couple of minutes later, and it's back on the Slacker screen and appears the last song playing was paused. Something is definitely wrong, either with the music app or related to the 3.5 mm jack on the Droid.