PS
For completeness:
colin@corundum:~$ grep unix_socket_dir /etc/postgresql/9.3/main/postgresql.conf
unix_socket_directories = '/tmp' # comma-separated list of directories
colin@corundum:~$
So despite pg_createcluster claiming that the default socket directory
for clusters owned by 'postgres' is /var/run/postgresql, it has indeed
used /tmp and as a result displays a warning.
Cheers,
Colin