You can use the Sun StorageTek Storage Archive Manager to create network mount points that provides advanced "Tiered Storage" services to Windows (via SAMBA) and NFS clients.
Tim Thomas has a blog entry, "What Does Sun StorageTek Storage Archive Manager Do ?", that expand on this idea.
A file can be created in the "fast" disk area then copied, based on policy, to "cheap" disk then inexpensive tape. Files are always backed up so there is no need for backups.