What's included in the default OSX 10.8 install of Postgres? I've seen several sites claim that Postgres is installed by default for OSX 10.7 and above and I see there's a psql command in /usr/bin/, but there doesn't seem to be a pg_ctl anywhere.

So is the default install of Postgres something that I can actually develop against or is it just there for OSX itself to use? I just don't want to install an extra version of Postgres and screw something up.
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)