Sammy.js does look quite cool. I've poked around the documentation, but haven't built anything with it yet.

I see what you're saying about "Javascript on Rails" but to me that's quite different than something like backbone. What you're talking about is more about "ajaxifying" a traditional browse-from-page-to-page experience. I definitely think there's value in that for improving the feel and responsiveness of a site. However, I'm more interested in truly emulating the desktop app experience where, once you log in there is no concept of browsing from page to page or "going back".