Encryption in PostgreSQL
This is a complex question and you will get a complex answer.

There is no reason you can't put a tablespace on an encrypted filesystem.  Additionally pgcrypto gives you a toolkit for doing data encryption but data encryption in the db is hard and there are too many ways keys can be disclosed to do this without review and assistance.  This isn't at the table level but rather at the value level, and pgcrypto gives you a plethora of options there.  Please see the pgcrypto documentation for details.