From PostgreSQL wiki
Synchronous replication is available starting in PostgreSQL 9.1 by enabling the synchronous_standby_names parameter. It includes user-controlled durability specified on the master using the synchronous_commit parameter. The design also provides high throughput by allowing concurrent processes to handle the WAL stream.
See also Synchronous Replication 9/2010 Proposal, though those notes pertain to a patch different than what has been committed.