Some members of the core team and I have started a "Rails API" project specifically to explore these possibilities, and to extract common patterns from the apps we build: http://github.com/rails-api

This will encompass hypermedia and non-hypermedia APIs, as well as SPAs with a Rails backend. Most of us are focusing on Rails 4 at the moment, and will start working hard on this post-Rails 4 release, but about 23,000 people have installed the main gem, and I know several running it in production already.

I personally feel the best option right now is "Rails API + ActiveModel::Serializers + Ember.js". But we want to encourage a multiplicity of options. Not all apps are identical.