What's included in the default OSX 10.8 install of Postgres?
Well, the fact that you have psql (the client) installed, usually means that you have the PostgreSQL server installed as well. However, it does not mean that the server is running. 

And, more importantly, OSX is an exception: 


  PostgreSQL is the default database on Mac OS X Server as of version 10.7. The standard version of Mac OS X includes only the PostgreSQL commandline client utilities. 


(from the documentation)