Migrating data can be automatic: https://github.com/crowdtap/promiscuous/

We use this tool to replicate data across our main MongoDB database, and a PostgreSQL one (eventually consistent).

We haven't wrote a comprehensive readme yet. The only documentation we have so far are slides: http://viennot.biz/promiscuous

Stay tuned :)

Those slides gave me a headache, so I'll just point out for the benefit of everyone that there is a README, and that it's some form of replication via AMQP queues.

Thanks for the tip on AMQP.