Streaming Replication
The other major replication related feature in PostgreSQL 9.0 is Streaming Replication. Once the standby server is synchronized with the master, this new feature will asynchronously send new WAL data over the network shortly after it's committed, in something close to real-time if things go well. This replication style is sometimes referred to as semi-synchronous replication. It's not fully synchronous, but the delay between commit and replication will be quite short in most cases; it's typically under a second, something impossible to accomplish with earlier versions. Expanding the database feature set to include fully synchronous replication instead when desired is in progress right now, and is expected to be included in the future PostgreSQL 9.1.