Yes. This is supported as SQL Server 2005 and 2000 can be installed side by side on a cluster successfully. With regard to achieve this there should be more care on choosing the shared- disks & IP addresses with sensible resourced hardware.
1. Separate Physically-shared disks for SQL Server 2005 and 2000.
2. Separate IP addresses for SQL Server 2005 and 2000.
3. If you install SQL Server 2000 first, when install 2005, please do not select the same instance name for 2005. Otherwise, setup will prompt for upgrading 2000 to 2005.
Read the complete post at http://sqlserver-qa.net/blogs/ha/archive/2007/08/10/is-it-possible-to-run-sql-server-2000-and-2005-version-on-same-cluster.aspx