After upgrading your Vsphere server to version 5 backups begin failing with "Argument is not in Datastore Format"
After the upgrade to vSphere 5 datastore names are changed to "datastorename"(1).
Rename the datastores back to there original name without the (1) at the end.
Once this is done you may end up with some duplicate names. Check each datastore for the existance of template files. If any templates exist delete them or move them to a different datstore. Once the templates are removed the duplicate datastore should also be removed.