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.
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.