WebDAV really rocks. I've been looking deeper and deeper into the spec in order to understand it's design principles (Jim Whitehead's excellent tutorial at OSCon last year clearly helped lots as well) and it's pretty obvious that a lot of good thinking has gone into it.
In fact it's looking more and more as if Tessera will be mostly an implementation of the DAV principles without the protocol, and little more than that. Well, little more that can be considered core bits. The rest is just built on top of that. Metadata, locking, collections, and objects: does one need more ? It looks as if maybe not.
In any case I strongly recommend that you give WebDAV a look if you haven't already.