upgraded ubuntu loses dovecot certs

ln -s /etc/ssl/certs/ssl-cert-snakeoil.pem /etc/ssl/certs/dovecot.pem
ln -s /etc/ssl/private/ssl-cert-snakeoil.key /etc/ssl/private/dovecot.pem

or:https://help.ubuntu.com/10.04/serverguide/C/dovecot-server.html

The other major issue I hit was the machine refusing to boot with a VFAT entry in /etc/fstab.  Remove these before the upgrade, or you'll have to do what I did and boot a live Linux disk (BackTrack 4 was what I had to hand) and fix the fstab file manually.