What is the first version of unity that supports failover with an exchange 2000 cluster? Where is this documented? Can the cluster be Active/Active?
Exchange 2000 clusters have always been supported with Unity failover 3.X failover. If you are looking for a version to deploy with, I would choose 3.1(5).
Active/Active clustering is supported. Check out the 'Exchange 2000 clustering' section of: