I'm bulding an ASP.NET website just to test my skills, and I'm using lots of callbacks that doesn't require a page refresh, and the URL doesn't change. In this example, assume I'm bulding a web-based Outlook with a treeview, a grid, and a detail pane.
Is there a standard (published or assumed) that says I should postback, or even update my URL from time to time?
The Standard you are probably looking for is called usability. DHTML, Ajax, or whatever you want to call it is fine until it breaks the users expectation of browser behavior. When the back button fails to work, and users can't bookmark the page exactly as they expect, you're doing it wrong.
I don't know about an official standard, but you may want to check out Gmail to see a good example of how something similar was done. The URL changes on the site much more often than the page refreshes.