Service Pack 3 for SQL Server 2005 has been Released. It is build # 9.00.4035
SP3 is based on all the fixes up to, and including, SQL Server 2005 SP2 with Cumulative Update 9, plus a few additional features (What's New in SQL Server 2005 SP3)
To see what fixes are included in SP3, see KB 955706
At the same time, Cumulative Update 11 for SQL Server 2005 Service Pack 2 has been released. It is build # 9.0.3301
CU 11 includes all the fixes released since Service Pack 2, excluding the new features introduced in SP3
At the same time, Cumulative Update 1 for SQL Server 2005 Service Pack 3 has been released.
CU 1 includes all the fixes released in Service Pack 2 CU 10 and CU 11 (SP3 is based on Service Pack 2 CU 9)
So we released SP3 which includes fixes up to CU 9, plus a few new features, and at the same time we released CU 11 for SP2 and CU 1 for SP3
CU 1 for SP3 brings SP3 to the same fix level as CU 11 for SP 2. CU 10/11 is intended to be used in situations where a hotfix is needed for an issue, but SP3 has not been approved for deployment in your organization.
So there are your options:
Cumulative Updates will continue to be issued for SP 2, until SP 2 exits support. A CU package does not replace a service pack and it’s installation is optional, and a CU is not offered through Windows/Microsoft Update or WSUS (Windows Server Update Services)
Service Packs are offered through Windows/Microsoft Update and WSUS.