PostgreSQL in OS X Lion
On OS X Lion "client", here's what you can find:

martin$ find /usr/ -name postgre*
/usr//include/postgres_ext.h
/usr//include/postgresql
/usr//include/postgresql/internal/postgres_fe.h
/usr//include/postgresql/server/postgres.h
/usr//include/postgresql/server/postgres_ext.h
/usr//include/postgresql/server/postgres_fe.h
/usr//lib/postgresql
/usr//share/devicemgr/backend/vendor/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
/usr//share/devicemgr/backend/vendor/rails/railties/configs/databases/postgresql.yml
/usr//share/devicemgr/webserver/gems/gems/eventmachine-0.12.10/lib/em/protocols/postgres3.rb
/usr//share/man/man1/postgres.1.gz
/usr//share/postgresql
/usr//share/postgresql/postgres.bki
/usr//share/postgresql/postgres.description
/usr//share/postgresql/postgres.shdescription
/usr//share/postgresql/postgresql.conf.sample


So no binary or similar on a default install (yet there are c/c++ headers) and rails stuff.  (note I have Xcode 4.1 installed, and this is where all these Header files may have come from).

2) OS X doesn't use a relational database for anything as far as I know. If it's there is to support development or, well, the service itself.