pgAdminのヘルプを読むと、以下の記述
For security reasons, a [[PostgreSQL]] server "out of the box" doesn't listen on
[[TCP/IP ]] ports. Instead, it has to be enabled to listen for TCP/IP requests.
This can be done by adding tcpip = true to the postgresql.conf file for
Versions 7.3.x and 7.4.x, or listen_addresses='*' for Version 8.0.x and above;