While PostgreSQL has had trigger based replication for a long time, it can be difficult to set up and maintain. Since 8.1 or 8.2, it has had warm standby, where you could fail over to a slave but not query it while replicating. The code for hot standby has been committed to the 8.5 development branch. From what I can gather from the mailing lists, 8.5 will probably be released around June.

http://www.depesz.com/index.php/2010/01/08/waiting-for-8-5-hot-standby/