As I was playing with my son - a great way to let the right brain chew over a problem - I came up with a different architecture for my memcache/CouchDB experiment, one that I think makes more sense.
Tempting as it may be ("hey, CouchDB is REST with JSON!"), I'm not going to expose CouchDB REST directly to the clients. In general I think it's dangerous (architecturally) to expose your database directly to clients.
Instead, I'm going to put a middleware layer on top so I can do clustering and caching, and also so I can swap in a different caching and storage mechanisms.
Another solution I'd like to try is Gigaspaces, but I'm getting ahead of myself...