Microsoft released Cumulative Update 6 for SQL Server 2005 SP3 yesterday (and the KB link is working today). This is Build 4266. There are over twenty fixes included in the CU, including a couple for database mirroring. All of the post-SP3 builds are listed here.

They also released CU 16 for SQL Server 2005 SP2, but honestly, isn’t it time you moved to the SP3 branch? SQL Server 2005 SP3 has been out quite a while now, and you will have to be on SP3 if you need to use SQL Server 2005 with SharePoint 2010.