psycopg can't find postgres process

Trying to connect with a pgsql 8 server (locally), psycopg return this error:

Traceback (most recent call last):
   File "<stdin>", line 1, in ?
psycopg.OperationalError: could not connect to server: ?R@
         Is the server running locally and accepting
         connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

There is nothing like postgres under /var/run/, so I'm wondering how
I can find out the the db server is running. Is this a known problem?
Could it have to do with the search path (I have to specify to
absolut path to start postmaster)?

best,
Christoph