Currently, PostgreSQL replicates a database WAL (change log) files to the slave databases, which made it more difficult to get slave databases to have less than a minute of lag. This wasn’t a big deal, since before 9.0, which is in development, you couldn’t query the slave databases until you failover, which stops replication, and that’s a rare event. With streaming replication, it will be possible for the slave databases to be as little as one second out of date, which makes them much more useful for offloading read-only queries from the master database.

http://www.depesz.com/index.php/2010/02/01/waiting-for-9-0-streaming-replication/