Built the XDelta 2.0 beta version to see if it was usable as a repository back-end. The documentation was very cool. XDelta 2.0 actually provides something like 70% of the server-side functionality modulo tagging and branch-management, but since it has metadata hooks, adding those features should be relatively straightforward. The feature set make the CryptoGnome look almost entirely trivial.
However, it would appear that the 2.0 beta (as of today) is not very stable. It dumped core on me just trying to create an object in it's DB. Plus, it's a pain to build, and the Berkeley DB embedded in it is not well-behaved wrt installation directories. So it looks like, as nice as it seems, XDelta is going to be on hold for a while