It depends. No, seriously. What is the clustering system in use? 'cos the Symantec / Veritas cluster you'll need to go through the process on all servers individually, whereas (as I understand it), you may be able to only do one of the active servers in a MSCS environment. (You'll probably have to shut down the other active node temporarily...) I have a clustering bloke here, and I'm trying to learn from him - when he's not too busy!
First of all, cluster administration is a tricky business - especially when running on anything before Windows 2008 and SQL 2008. Before doing anything: read, understand, *practice*, prepare (incl. backup), then implement. I agree with Thomas that you have to be careful depending upon environment. As I understand it, you install the SP on the active node, and it takes care of the other nodes. I think you need to fail service over to one of the nodes after reading the information below. As found in the SP3 Release Notes from Microsoft: Installing SQL Server 2005 SP3 on a Failover Cluster Instance Note the following information before you install SP3 on a failover cluster instance: - Do not stop the cluster service before you run the SP3 Setup program, or while the installation program is running. - Do not end any running processes before you run the SP3 Setup program. - Do not take the SQL Server service offline before you run the SP3 Setup program. The SP3 Setup program will stop and start the SQL Server service. - Run the SP3 Setup program on the primary node of the failover cluster instance. - The SP3 Setup program might require you to restart the failover cluster nodes. This restart replaces the files that were in use during Setup. - Rolling upgrade is not supported for SP3. For more information about failover cluster installation, see the ["Installing the Service Pack (Hotfix)"] topic in the SQL Server 2005 SP3 Setup documentation. The Setup documentation is available from the SQL Server 2005 SP3 Readme Files download page. :