I've got a bug report up on Launchpad describing this issue in detail. See here: Bug #794112
The gist is that I'm using LDAP and Kerberos to authenticate NFSv4 mounts on my clients according to a configuration described at http://www.danbishop.org/2011/05/01/ubuntu-11-04-sbs-small-business-server-setup-part-3-openldap/. The problem seems to be that when users go home for the night their Kerberos tickets expire, and the client OS gets thoroughly wedged on the missing NFS volume.
From the client logs:
"Jun 6 10:53:28 carina kernel: [47636.670075] Error: state manager encountered RPCSEC_GSS session expired against NFSv4 server 192.168.0.2."
It's hard for me to believe that such a common scenario should have to result in a cold reboot every morning. This absolutely can't be the intended behavior.
Does anybody have a clue what's going on?