The intended use of CouchDB in UbuntuOne seemed to be tailor fit with the technical design. (Distributed master/master replication and resolution, eventual consistency etc)

I'd love a write up with more technical details describing the issues.

While it's not firsthand, this is a good summary: http://news.ycombinator.com/item?id=3276466

In short, it boils down to problems around authentication while sharing by means of replication.

I agree. The whole thing sounds like "It should have worked but, for an unspecified reason, didn't."

Unfortunately, I wouldn't be surprised if it never really comes out. Company proprietary information and all that.

Aren't Canonical and CouchDB all open-sourcey, community-first etc etc etc? Then why the secrecy here?

To me, it sounds like couchdb just failed to scale in a big way and revealing how would basically kill the project, so Canonical are trying to do the polite thing.