Apache CouchDB developers respond to UbuntuOne issue
h-online.com
h-online.com
I'd love a write up with more technical details describing the issues.
Unfortunately, I wouldn't be surprised if it never really comes out. Company proprietary information and all that.
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.
In short, it boils down to problems around authentication while sharing by means of replication.