Automated Database Backups

Does anyone have a way to run pg_dump from cron in a way that doesn't
require someone to enter the password on every run?  I get the following
error even when the backup user has read permissions on all application
databases.

The database that fails is part of the PostgreSQL kernel, not my
application data so I'm leery of tampering with permissions here.

The command line I am using is:

/usr/postgres-8.4.2/bin/pg_dump -f /notraid/dbbkup.`date +%Y%m%d` -F
custom -v -U xxxx dbxxxx

Thanks in advance.

John