Did you upgrade from, say, 8.3.3 to 8.3.5?
Newer rc script can be found here (look for "rc.postgresql.new"):
http://slackbuilds.org/repository/12.1/system/postgresql/
At the least, change from / to:
su postgres -c 'postgres -D /var/lib/pgsql/data &'
su - postgres -c 'postgres -D /var/lib/pgsql/data &'
Also check:
$ sudo cat /etc/passwd | grep postgres
postgres:x:88:88::/var/lib/pgsql:/bin/bash