Due to the long term nature of archival backups it is not appropriate to tie them to a particular filesystem format, and so a scheme similar to the Calamity backups would not be appropriate even if it were practical.
A cpio or tar archive of the home directory is likely the best option for these backups. The main burden that this imposed on a filesystem is the fact that files are normally collected sequentially and there is no opportunity for read-ahead between files. A better approach would involve submitting read requests for multiple files and directories in such a way that throughput is valued over latency, and the order of the returned data is not important. Then the filesystem could submit large number of requests to a device before waiting for any of the data, and so provide lots of opportunity for re-ordering and grouping requests and thereby improve throughput.
It may be possible to use the new asynchronous IO primitives in 2.6 to achieve this improved thoughput. If not a special purpose interface to the filesystem will be provided to provide optimal access for backups.