While looking through the port manifest for Postgres 8.0 on quarkprime, I noticed that the FreeBSD package contains scripts that automate daily vacuuming and backups of the database. I've upgraded quark from 7.4.7 to 8.0.1. I had to do a pg_dumpall and then a restore to do the upgrade because of the major version change.
These are enabled by adding these two lines to /etc/periodic.conf:
daily_pgsql_backup_enable="YES"
daily_pgsql_vacuum_enable="YES"
The backups are done on individual databases using pg_dump, and are plopped in /usr/local/pgsql/backups. Because periodic stuff is run at 3AM, I've moved the amanda invocation from 3AM to 4AM.