Eu tive um problema semelhante no meu novo MacBook Pro executando o Mountain Lion. Baixei o PostgresApp e quando acionei o Rails recebi o seguinte erro:
`initialize': could not connect to server: No such file or directory (PG::Error)
Is the server running locally and accepting
connections on Unix domain socket "/var/pgsql_socket/.s.PGSQL.5432"?
Para corrigir isso, tive que definir explicitamente o host e a porta no meu
database.yml arquivo assim:development:
adapter: postgresql
encoding: unicode
database: my_app_development
pool: 5
username: tony
password: stark
host: localhost
port: 5432
