That's pretty interesting and didn't realize anything like this until i read this thread...
I listen to A LOT of Slacker Radio on my phone, but will occassionally listen to mp3's. (I live in NYC and can't stream Slacker in subway tunnels). Anyway, the other day I was listening to Slacker streaming to my phone but playing through 3rd party speakers in the morning while I was showering. Then I got dressed, hit "pause" on Slacker, walked to my front door, plugged my headphones into my phone, hit "play" on Slacker and when I put my headphones in my ears, the mp3 player had started also -- I could heard Slacker and one of my mp3's being played at the same time. I figured it was some kind of goofy glitch because this has only happened once so far but it would make perfect sense that the default mp3 player kicked on when i plugged in the headphones.
Fabolous, since you asked: I was using a pair of $15 piece-of-shyt Sony headphones that wrap around my ears. This morning I actually bought a pair of Skullcandy ear buds but haven't tried them out yet. I'll report back if I get the same behavior with the new ear buds.