A web app where this is just one HTML file but the JavaScript gives it the feeling of being a whole site (client side template rendering, etc). I think in essense a single page app is one where there is no server side temp laying.

What do you think of the single page app you've described with multiple html entry points?

Say / responds with the homepage as per usual and /feed responds with a different html start point, but the same application code.

Using this technique you could create a site that responds to multiple urls with html and at the same time creates a SPA experience using a single js application (say, with backbone for example).

It's really the Rolls-Royce solution by which I mean it's a better but a lot more work. I didn't have that amount of time to commit. :)