Available now, from The MySQL developer community, is the Milestone release, 5.5-m2. A quick rundown of the release notes include semi-synchronous replication, enhanced partitioning syntax, and SIGNAL and RESIGNAL methods to programmatically return errors. Additionally the InnoDB Plugin is included is included in 5.5 as the built-in version with performance, scalability, and reliability improvements. And, as with most new versions, there is the added deprecation of syntax and bug fixes.
You can look at the Milestone 5.5-m2 download page for more information.