I was reading node.js for fun today and I decided to create a simple websocket chat. I've used socket.io (both for front and the backend) and the express framework, which is similar with sinatra. I may add redis in the future for consistent data. Also I haven't implemented the current logged in users, I have to attach two events for handling it. It isn't difficult, it is a nice exercise if you also want to experiment (and don't forget to do a pull request! :)). Anyway I think this chat can easily be added to a rails app with some minor customization and play nicely. Hope you like it.
Node.js isn't that bad for what it does, I'd like to test some more things. Until now, when I wanted a realtime app with web sockets, I was using pusher, which is really easy to setup and use it inside a rails app. Now I may try node.js as well.