Single Page Applications: something to watch?

I discovered Trimpath Junction today, a MVC javascript bringing RubyOnRails' convention over configuration to javascript. But what I really found interesting, is their Next Action demo available as a Single Page Application. The concept: with Firefox, you save the page on your disc, and you have the application on your disk. After you edited or added data, you save the page again and the application with the updated data is saved.

Of course, there are downsides. Isn't one of the advantages of web applications their centralisation and their remote availability? Anyway, I find it an interesting concept. Interesting enough to keep an eye on to see how it evolves.