Exchange 2010 clusters can be protected by multiple DPM servers
The DPM Team blogged about Exchange Server 2010 DAG protection and DPM where nodes can be protected by different DPM Servers.
I like to draw attention to something that may not be well known and surprise some. You probably are familiar with the 'rule' that all protected hosts within a cluster must be protected by the same DPM server! This is does not apply to Exchange 2010 and is formally documented here: http://technet.microsoft.com/en-us/library/ff399324.aspx.
You still get the warning that all nodes must have a DPM agent installed but can be safely ignored for Exchange 2010. In that context you may also want to take a peek at a related blog on resolving alerts; http://blogs.technet.com/b/dpm/archive/2010/06/04/automatically-resolving-alerts-such-as-resume-ownership.aspx
With other workloads including Exchange 2007 CCR the application store is considered to be shared across nodes in the cluster. DPM protection is not per definition fixed to a single physical node (although recommended for Exchange 2007 CCR). To maintain a single view on recovery points for a data source that from time to time could synchronize data using different nodes, these must belong to the same DPM server.