In a javascript only web application, what state information should the URL contain?

Question

I'm writing a single page web application that talks to a business layer via asynchronous RPC (encoded with json). I'm targeting fairly modern browsers, so I can at minimum control the URL after the hash. However, the URL won't really be what drives the application's state (that will be maintained by a javascript model layer), so I don't have to do anything with it. I could even keep it the same the whole time without any implementation problem.

The question is, what should go in the URL? I won't go into the application because I'm hoping the question is generally useful. What kinds of expectations do users have about backward and forward buttons? About typing in a URL? Are there currently any generally applicable best practices yet for this kind of application?