The default /var/lib/pgsql/data/postgresql.conf has #tcpip_socket = false and must be changed to tcpip_socket = true to allow TCP/IP connections, even from applications on the localhost. This is rather annoying, and difficult for non-technical users to do when installing applications such as Glom. By the way, debian activates this by default.
This is following the default upstream behavior, which I'm disinclined to have the RPM editorialize on in any significant fashion. Loosening the connection restrictions from what users expect from the PostgreSQL documentation could result in unexpected security holes. FWIW, Postgres 8.0 does allow local TCP connections by default, so FC4 will act the way you want. But I think it's a bad idea to change it in FC3.