pg_tablespace_size()

Following on from Hiroshi's earlier post:
http://archives.postgresql.org/pgsql-hackers/2007-10/msg00324.php

Per http://archives.postgresql.org/pgsql-hackers/2007-08/msg01018.php,
superusers should be able to use pg_tablespace_size() on any tablespace,
however when logged in as postgres on a beta1 server, I get:

-- Executing query:
select pg_tablespace_size('pg_global');

ERROR:  permission denied for tablespace pg_global

Can this be fixed please?

Regards, Dave