Thanks to Tim O'Reillys tweet for this.
Imagine building an app that that can run standalone just as easily on your laptop or on a cloud-based cluster.
Such a thing may be possible using AJAX and a CouchDB backing store running on the same machine as your browser or serving up the app and data from a server. No PHP or Ruby middle tier getting in the way.
CouchDB provides replication for free, so all of your data can be replicated to your server or to any other peer. It's the framework for true offline, peer-to-peer data sharing applications.
Oh, and did I mention it's logarithmically scalable? Very cool.