When you perform a backup, make sure the Cluster Shared Volumes disk is in redirected mode. Backup software that supports Cluster Shared Volumes does this via a specific backup call to the Cluster Shared Volumes storage, which allows the Failover Cluster manager to show the disk as "Backup in progress, Redirected-access." This lets administrators looking at the console see why the Cluster Shared Volumes disk is in redirected mode, in addition to actually putting the disk in redirected mode.
When looking for backup software, make sure it states it supports Cluster Shared Volumes; System Center Data Protection Manager 2010 and BackupExec do (and there are many more). Also, to ensure the best performance backup, if you have hardware that supports Volume Shadow Copy Service (VSS), make sure you have the VSS hardware provider installed on all nodes in the cluster.