What would you use it for? Also, how would one go about implementing something like this in practice?

CouchDB isn't truly distributed, so one would need to run multiple isolated instances which would in turn impose restrictions on the size of a single database as well as the number of concurrent writes.

I've been toying with the idea of writing a CouchDB interface on top of HBase (or possibly some other column oriented DBMS) to overcome these issues and am wondering if anyone else has considered doing the same thing.