When I turn ot on I want to be mounted in /mp3 and preferable open a window in gnome with it's contents.
AFAIK this is not one but two things: automount stuff and show filemanager in Gnome.
For the automount there's hotplugging:
...or Udev with custom rules:
And for the Gnome-open-window thing I don't know if it picks up the mount changes (FAM, Dbus or whatever else). If it doesn't, I do know it works with HAL (the horror, the horror...): just needs some custom rules to mount it under a name you want. Apart from the mind-blistering horror of having to add yet another service plus a cartload of dependencies for something as simple as automounting devices the (big) difference is that with Udev you can make it run scripts which allows for way more fine-grained control over whatever you want control over.
Something I haven't figured out yet how to do with HAL.
Just my thoughts.