How can I stop iTunes 10 launching when pressing the Play/Pause buttons?
Just found this on Lifehacker.  I think it's exactly what you're looking for.  The post talks about a patch called MMFix, which changes the media control daemon to stop exactly this from happening.  Apparently, it comes with a one-click installer.

And, if some reason you would like to remove the patch, just run the one-click installer again and the original rcd daemon will be restored.

(Works on OS X 10.7 Lion, too!)