I've upgraded image, backup, and proto to kernel 2.4.25. No problems detected thus far.
In the migration to the new quark, MIMEDefang stopped passing mail through SpamAssassin.
To solve that problem, I setup a separate milter for SpamAssassin. I installed the spamass-milter package, and added this line to quark.cs.earlham.edu.mc:
INPUT_MAIL_FILTER(`spamassassin', `S=local:/var/run/spamass-milter.sock, F=, T=C:15m;S:4m;R:4m;E:10m')
After restarting sendmail, everything was working fine.
(Check to make sure it starts on boot-up)
I have enabled SSL on bagend, giving encrypted access to Nagios.
I had to setup a virtual host in ssl.conf, and generate new SSL keys to reflect the actual DNS name of the machine.