createuser: could not connect to database postgres: FATAL: password authentication failed for user "postgres"

Hi List

This question has already been asked many times, but I didn't find the
answer I'm looking for, so here goes

I use postgresql 8.3 on suse 11.2

I already installed postgresql many times wit postgis support on
debian/ubuntu machines. Never had problems, now I'm installing it for
the first time on suse linux. After installing, I changed the following
in postgresql.conf for a production environment
shared_buffers=128MB
checkpoint_segments=20
maintenance_work_mem=256MB
autovacum=off

And then I restarted postgresql and switched to postgres user, and then:
"createuser gisuser"
After typing password the console gives the error message "createuser:
could not connect to database postgres: FATAL: password authentication
failed for user "postgres"

So I change the user postgres password using the command passwd as root
and I give in the same password and I repeat the createuser command (or
createdb whatever), only to get the same error message

Can someone see what the problem might be?

Best regards
Moataz