Specifically CouchDB. Not SimpleDB or Google's Big Table. I've played with CouchDB and Python and I like it but it's overkill for my tiny just for fun projects, but if it turned out more cost effective to host in-house in the future, not having the lock-in is super attractive.

If CouchDB is overkill, how simple are your projects? I thought CouchDB is supposed to be as simple as it gets?

to clarify: too simple to justify upgrading my host and installing when mysql is a button push away.

I like the concept. There could be an interesting business idea here. One of the problems that concerns me is speed of data access. If I'm running the service on AWS, your app will need fast access to your data. If you are hosting your app on AWS, why not run couchDB yourself? You will get much better speeds utilizing the cloud service's internal infrastructure rather than accessing your data via the internet. If you are hosting your web service somewhere else, then why not use the database solutions provided there? I'm just trying to understand the need for this kind of service.

You could run standalone apps made up of static JavaScript, HTML and CSS where the browser clients talk directly to CouchDB. Check this out: http://books.couchdb.org/relax/hosted-applications