At MySQLcon I ran into the guys from Codership. They did a 25 minute presentation on how to do multi-master synchronous replication across many instances of MySQL.
In layman's terms.
If you are running 5 Master MySQL databases behind your giant Drupal site, and one of those servers crashes, you won't lose a transaction, and you won't corrupt your logs and destroy your database.
Today Drupal.org runs 2 multi-master databases, but if one of the DB servers crashes we could lose a transaction, and we could have our binary logs get corrupted. Fortunately, we have back-ups and we don't do financial transactions on the Drupal.org infrastructure. General availability is targeted at the end of 2009. They are working to get hooks added to the MySQL core.