If I understand correctly Can you mix versions of SQL on a cluster? Yes How? Use named instances. Create your cluster, setup the drives etc Pick which to install as a named instance (or both) Then install as you require
You can run both 2005 and 2008 versions on each node of a cluster, but you cannot failover between versions. Picture this scenario: Server A is the active cluster node for 2 instances: SQL2005 and SQL2008 Server B is a failover node for both instances. If Server A fails, the SQL2005 instance would failover to Server B, and the same would happen with the 2008 instances. But you must configure the clustering for each instance, and both servers will need to be configured for both instances (using the respective versions). I hope that answers your question.