Comment 22 for bug 326988

I ran into this same system similar after upgrading to Jaunty on a system that uses autofs and network-manager. The problem was that network-manager by default does not share the connection, so when the user logged out, networking stopped and any network filesystems that were mounted when the user logged out would cause shutdown to hang. The workaround was to check 'Available to all users' for the network connection in network-manager.