Yes we have it in Version 3. 🙂 No it's not in the Implementation Guide. The instructions are available online now.
You can install Microsoft Dynamics CRM 3.0 Server into a clustered Microsoft Exchange or SQL Server environment. A cluster of servers running Microsoft Exchange or SQL can reduce system downtime because if the active server in the cluster fails there will be an automatic fail-over to another server.
Ohh yeah, this only works with the professional edition. (I don't think Small Business Server allows for clustering... 🙂 )
And in case you wanted to know how to setup an Exchange Cluster... Here you go.
And in case you wanted to know how to setup a SQL Server Cluster... Here you go.