If you've thought about using SQL Server 2005 Upgrade Advisor to determine the upgradability of your SQL Server 2000 cluster, be aware of the following issue. When you launch the Upgrade Advisor Analysis Wizard, you're prompted for the Server Name, which defaults to that of the local machine where the wizard is running. When scanning a cluster you need to enter the name of the Virtual SQL Server Instance. After checking the SQL Server component and clicking the next button, you'll be given a list of instances to scan.
The list of instances will include all SQL Server instances, whether they are local to the active node, or if they belong to other virtual SQL Server instances that run on the cluster. Selecting an instance other than the instance owned by the Virtual SQL Server cluster will ultimately raise an error when the wizard attempts to connect to the server and enumerate the list of available databases. To avoid this error, be sure to select the appropriate instance for the "server" you're connected to.