unattended backups

Greetings pgsql gurus,
  I have a pgsql database that uses md5 passwords.  When I want to back
it up the "pg_dumpall >dump_file" command prompts for the administrative
password, evidently via stderr.  I'd like to stick a "pg_dumpall" in a
cron job, and supply it with the password.  I don't mind having the
administrative password somewhere readable by root, but I am stumped how
to carry this out.  Has anyone been down this road ahead of me and found
an answer? (other than "abandon hope ...")
cheers,
Andrew