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... :-) )
Click Here for the instructions for Exchange
Click Here for the instructions for SQL Server
Click Here for instructions on "clustering" Microsoft CRM
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.