There's a really interesting video with @dhh talking about his thoughts on client-side MVC frameworks with Rails.
www.youtube.com/watch?v=FkLVl3gpJP4

He goes into some detail about Pjax and 37signals caching strategy so that partial page updates are snappy and that gives you an experience much like the client-side frameworks without the added complexity.

After working with Backbone.js for a while now I agree with him that development experience of client-side views isn't nearly as nice as generating the views from rails.