MySQL released version 5.1.55, this is a bug fix release which also includes some security fixes and it is up to you if you want to upgrade. Check all the fixes and changes that are listed on the release notes to see what issues are resolved and if you are affected. I’ve compiled it and did some tests on my servers and it worked without any problems.
Read the documentation on how to ugrade this version of MySQL.
MySQL also release a new version of the 5.5.x branch which is already usuable in our setup when you read this blogpost. It works perfectly on my test machine. It includes several high-impact changes to address scalability and performance issues in MySQL Server. These changes exploit advances in hardware and CPU design and enable better utilization of existing hardware. MySQL 5.5 also provides a number of other new enhancements. Read the release notes for more details.
Read this documentation on how to ugrade this version using the package installer of MySQL.