I have a very similar issue. I'm almost 100% sure the issue is I have two volumes with the same name. They are on separate storage systems, so it's legal, but VCS appears to be getting confused. I noticed this entry in the log file:
2012-01-13 19:00:07,731 [backup:997072497839aa6396e2acc8825af7cc:] WARN - Unable to manage datastore vmware_vol1: Unable to discover lun xx:xx:xx:xx:xx:xx:xx:xx@6 mapped to initiator xx:xx:xx:xx:xx:xx:xx:xx used by datastore vmware_vol1 (4f10764f-346b670d-deaf-002481802e04)
com.netapp.smvi.task.vmware.util.InvalidDatastoreException: Unable to discover lun xx:xx:xx:xx:xx:xx:xx:xx@6 mapped to initiator xx:xx:xx:xx:xx:xx:xx:xx used by datastore vmware_vol1 (4f10764f-346b670d-deaf-002481802e04)
2012-01-13 19:00:08,121 [backup:997072497839aa6396e2acc8825af7cc:] ERROR - FLOW-11008: Operation failed: Did not find any of the following datastores or virtual machines to backup: []. Storage system(s) may need to be added.
The initiator specified is mapped to the storage controller NOT involved in the backup. I chose a VM target to back up (not a data store). I'm guessing there is something wrong with the discovery process where VSC tries to determine what to back up based on what VMs you choose.
I verified this was the issue by renaming one of my volumes to something slightly different.
Hope this helps someone down the road.