In MySQL 5.5 replication can optionally be made semi-synchronous instead of the traditionally asynchronous replication.
The clients COMMIT (or in auto-commit mode the current statement) waits until _one_ slave acknowledged that it received (not neccesarilly executed) the transaction or a timeout is reached. In case the timeout is reached, semi-sync replication is disabled.
To see of the master supports semi-sync replication run:
SHOW VARIABLES LIKE 'rpl_semi_sync_master_enabled';
The slave requests semi-sync replication by sending:
SET @rpl_semi_sync_slave = 1;
1 [ef] semi-sync indicator 1 semi-sync flags