I dig this. I've been using Ember + Rails and it still feels like client-side frameworks are reinventing a lot of server-side wheels because you still need a server-side layer for security, optimization, etc. I've been adding a lot of shims to "just work" in the Rails-world, and having a tightly integrated JS framework for Rails seems like a fantastic idea.

Models on the client are likely to be slightly different (I wouldn't do image resizing on the client, for example) and so the one-size-fits-all approach (Node.js Models running on server OR client, magically) doesn't work either.