Defaults and reset values
The database has two things that you might refer to as a "default" depending on your context. The first type of default is what the server will set the value to if you don't ever change it—the setting the server starts with before it has even read the postgresql.conf file. Starting in PostgreSQL 8.4, you can check this value using the pg_settings view by looking at the boot_val column: http://www.postgresql.org/docs/current/static/view-pg-settings.html.
Once the server has started, and you make changes to parameters there's also a default value those parameters will return to if you use the RESET command documented at http://www.postgresql.org/docs/current/static/sql-reset.html to return them to their starting value. This is labeled as reset_val in the pg_settings view.
