I've written a little tool, VirtSync http://www.barricane.com/virtsync for this purpose. I use it to synchronise VM disk images. It currently serves my needs, but is a little hacky in places. I want to know whether it there is a viable market to make it worth cleaning up and packaging.It is based on Hash Trees, http://en.wikipedia.org/wiki/Hash_tree so it only copies the deltas and it does in-place writes.
(rsync doesn't let you use --inplace and --sparse together.)