rsync and .zfs snapshot directories

zfsonlinux recently added support for visible .zfs snapshot directories, so you no longer have to mount a snapshot to browse/view/recover files from it. This is good, but has caused my rsync backup to zfs script to complain about being unable to delete files in the .zfs directory (which is in the backup but not in the source). e.g. sending incremental file list rsync: delete_file: unlink(.zfs/snapshot/2012-03-31/vmlinuz.old) failed: Read-only file system (30) rsync: delete_file: unlink(.zfs/snapshot/2012-03-31/vmlinuz) failed: Read-only file system (30) […] cannot …