iTunes stops playing when you switch users under Mac OS X. This has been the case for rather long.
I agree that this is the most sensible behaviour. After all, the user that has switched herself in might want to use her own iTunes and music.
However, there are still valid use cases where you'd like iTunes to keep playing. I have two: I personally have several users for different kinds of activity. Or even when I am together with my SO, we share the same iTunes library and choose together. So when she logs in to her account, she still wants my iTunes to keep playing. And the other way around.
So whether this is appropriate or not, is there a way to trick iTunes into keeping playing when the user account is switched out ? (not logged off of course).