It depends on how you define OS, a media player is not really part of the OS, so it could very well be closed source. It could also be that Manufacturers add other proprietary parts (such as drivers, power management code) when they create a release for a specific device. I hope this will not be the case and Nokia will strive to make a good example and open source as much as possible when they integrate the current Maemo 5 into MeGoo.