CouchDB doesn't "scale"? If you're trying to "scale" with it, you don't know what you're doing in the first place. CouchDB federates. That's a wholly different thing. And in terms of federated databases, I challenge anyone to come up with one as good or better than CouchDB. (And if you do, it will be news to me, and I'll thank you profusely!)
If its not obvious to you how to scale a federated database, then its not couchDB that can't scale, its you. (which is ok, everyone has to learn sometime, just don't put forth your lack of knowledge as proof of a weakness in an open source product!)
Further, rather than just saying "We've got this great new invention-- a better technology, and we're moving to that!" the message seems to be "we are just wanting to re-invent the wheel, so to justify it, we have to make a negative claim about couchDB.
Now, I expect some particular databases[1] fans to tell us, in the future, that "couchDB doesn't scale".
Ironically, they're punting on CouchDB to use, among other possibilities, SQLite. To claim that "Scaling" is the problem is .... bad engineering form.
CouchDB is great if you want to federate, have databases across the planet talking to each other and keeping in sync (its almost a turnkey CDN in a way), want to run a noSQL DB on a mobile device, etc.
MongoDB is great if you care about SQL and single node performance and its complex distribution mechanism works for you.
IF you want "scale" your choices are Riak or CouchDB-- for "scale" where homogenous distributed servers are the best solution.
And of course there's Cassandra and graph databases, etc. which provide different solutions to scalability.
IF you're serious about scalability, I strongly recommend people look at and choose Riak. I don't think anything out there touches it-- at least for the type of data I need. Cassandra and what I consider the "more complicated" alternatives might fit your particular problem type well. And if you think that its silly of me to recommend Riak then this is probably the case for you. But in terms of general databases, Riak seems to be pulling away from the pack. IF you're a fan of CouchDB, then BigCouch is a dynamo/Riak like version of it that I understand to be quite good. Plus, since its based on CouchDB, if the CouchDB way of doing queries (which is distinctly different from Riak) fits your way of working, then BigCouch deserves a look.
But please, don't ever say "couchDB doesn't scale". If you do, really its that you don't scale, CouchDB is fine.
[1] In an earlier edit I named a database. That was a mistake, not only is it bad form, I don't think that my characterization is appropriate at this time, as that database's fans are not as rabid as I imply. Apologies.