Cloudant sits under the Apache CouchDB API, so the easiest path for migration in/out of Cloudant is to use replication to/from another CouchDB-speaking service like Cloudant, Apache CouchDB, TouchDB (mobile), PouchDB (browser). For most databases, you can also get a JSON dump of the document bodies by doing: curl 'https://username.cloudant.com/dbname/_changes | gzip > dbname.json.gz.

The tricker part of course is understanding how to best model data as you move between different database systems. There are some nice tools emerging in the no/new-sql ecosystem to help with that. Here's one that some of our mysql/postgres converted customers seem to like: http://www.foxweave.com/moving-on-premise-data-to-the-cloud-.... My understanding is that it works in both directions.