Adrian asks the correct questions. Lacking the answers to those I'm going to venture a guess that a Unix-domain socket exists but access via Unix-domain sockets is somehow blocked, probably by pg_hba.conf.
From the psql man page: "...Not all of these options are required; there are useful defaults. If you omit the host name, psql will connect via a Unix-domain socket to a server on the local host, or via TCP/IP to localhost on machines that don't have Unix-domain sockets...."
Cheers,
Steve