Working across multiple PostgreSQL database servers
Foreign data wrappers are currently read-only.  This will change in the future, but that's the way it is.  You can also use dblink to access other databases.  As I understand it a big issue will be commit handling.  I would recommend against this approach unless you are trying to aggregate lots of data that is really external.  It is a huge complexity cost and the benefits have to be worth it.

Edit:  Note that foreign tables and foreign servers are how you use foreign data wrappers to map in foreign data to your application.  You can't choose between them.  It's all three or nothing.