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I am glad you wrote about this, but I feel you are missing the real 'magic' that can be done with node.js and Backbone, or any client code you choose. Socket.io is great, fantastic even, and I think it is something that really shows the power of node.js; there is something built on top of socket.io that is even more wonderful, and it is called dnode[0]. It is really simple to use, but has a lot of potential for server process to server process and server to client communication. It is in effect RPC, and I really can't do it justice.
For the example you give up here, you could easily overwrite the Backbone.sync function and have it instead send the appropriate objects you need to fetch models on the server, while having the server call back to the client with said models. Not only that but you could easily remotely invoke the model changed functions in Backbone directly from the server. There is so much more you can do, but I lack the knowledge at this point to put it in play just yet.
[0]: https://github.com/substack/dnode & http://substack.net/posts/85e1bd/DNode-Asynchronous-Remote-M...