Installing WSUS 3.0 SP1 as a Role on Windows Server 2008 SP2
The WSUS Product Team posted instructions on their Blog site describing how to install the WSUS 3.0 SP1 Role on a Windows Server 2008 SP2 server. This is for a scenario in which you have a Windows Server 2008 machine which does not already have WSUS installed, and you either already installed Service Pack 2. Once SP2 is in place, you can install WSUS as a role using the Server Manager console. Read their notes and comments for more details.