Microsoft has released CU8 for SQL Server 2005 SP3, which you can get here. This is Build 4285. This CU has five listed fixes, including two for database mirroring. Interestingly, there does not seem to be a “matching” CU for SQL Server 2005 SP2 (which is fine with me). People need to be getting off of the SP2 branch by now.
Bob Ward explains Microsoft’s support policies for SQL Server 2005 and 2008 here.