Android - How can I stop the Music app from launching when headphone is plugged in?
It's been reported as a known issue on code.google.com. There's a long list of 79 comments which seem to indicate that rooting and adding a program called "Autostarts" fixes the problem. This is obviously not something everyone is going to want to do... be careful if you go that route.
You can use the Headset Blocker widget to disable that behavior.