psql -U postgres does not require a password

I've just upgraded Redhat RPMs from Postgres 6.5.3 to 7.0.2 on two servers
and run into the same problem on both. When I call the client psql:

psql database_name -U postgres

I'm not asked for a password and every user can access the database with
postgres privileges.
The password for postgres user is set (if I do a "select * from pg_shadow" I
can see it). In addition this happens with every user, not just with postgres.

What am I doing wrong ?
Thanks.

Silvan