Bypassing authentication

Is there a way to have psql bypass authentication as joe if the
system user running psql is postgres, the way su doesn't ask for
password if you are root ?

sudo -u postgres psql -l         # psql doesn't ask for a password
sudo -u postgres psql -u joe -l  # psql asks for joe's password

joe is just a PostgreSQL role with no system counterpart so the
"local all all ident sameuser" catch-all in pg_hba.conf is of no
help here.

Thanks in advance.

--
André Majorel http://www.teaser.fr/~amajorel/