Currently pulseaudio keeps sound devices open during its entire lifetime, making it impossible to use sound in the second session. For our fast user switching effort, we've changed things so that devices now have acls that allow all users with active sessions access, but that doesn't fix the problem with already opened devices (and the kernel doesn't have revoke). The short-term plan is to make pa react to ConsoleKit signals about the session being deactivated, and voluntarily close devices then. Monty is looking into this.
This is now fixed in Rawhide. PA now listens for HAL ACL changes and acts appropriately. There still seem to be some bugs with HAL not setting ACLs always properly. But that's a HAL bug (dn't know which bz number right now), not a PA bug, so I am closing this one now.