Single Copy Clusters are the continuation of how clustering was implemented with previous versions of Exchange.  It is a standard cluster implementation which utilizes Shared Storage (i.e. SAN) and allows all nodes of the cluster access to the shared storage.  One node of the cluster will "own" the resources for that cluster group, and all nodes of the cluster maintain communication to ensure that the resources don't get mistakenly taken over by another node.  A quorum is used to maintain ownership information.

That being said, Exchange 2007 does make some slight changes to how you must implement the cluster.  I do want to note that setting up a cluster in Exchange 2007 is MUCH MUCH easier, but there are a few *gotchas*.  First, if you are familiar to clustering with Exchange 2003, you may be inclined to pre-create the Cluster Group for the Exchange Virtual Server, and pre-populate it with the Physical Disks you will be using.  Don't.  Instead, all you need to do is create your Physical Disk resources in the default Cluster Group.

An overview of how to install Exchange 2007 Single Copy Cluster on Windows 2003 can be found here.

http://technet.microsoft.com/en-us/library/bb124899.aspx

and for instructions on how to install the Active Clustered Mailbox Role in a Single Copy Cluster, refer to

http://technet.microsoft.com/en-us/library/bb123969.aspx

Once you have finished the Active Mailbox role, you will find that Exchange 2007 setup will create the new Exchange CMS group for you.  The only step that remains is that you need to finalize configuration of the cluster by moving the physical disk resources from the default Cluster Group into the new Exchange CMS cluster group, and then configuring the dependencies as appropriate.  In Exchange 2007, you will have a separate resource for each Storage Group/Mailbox Store, so it is important to make sure that you get the dependencies correct.