Windows Server 2008 Service Pack 2 (SP2) is now available. This post will walk you through performing a “rolling upgrade” from Windows Server 2008 RTM to Windows Server 2008 SP2. A rolling upgrade allows you to progressively upgrade your cluster nodes while keeping your resources highly-available, however you may have some downtime while moving the resource groups between nodes. What happened to Service Pack 1? Windows Server 2008 RTM was actually SP1 as it was released around the same time as Vista SP1.
Full Article: MSDN Blog