Unless you get serious about AJAX (many people bet on this).
While you can certainly craft stuff on top of AJAX that behaves like a callback system, it's still running on top of HTTP, i.e. request/response pairs originating from the browser.
That means you're still just polling for data from the client as soon as you need any information from the server side. You're not pushing data to the display as you do in traditional GUI applications, where the view also knows a lot about the state of the display. Actually, my guess is that the more AJAX technology you're using, the more you'd have to focus on plain output & http caching if you want to get any kind of performance (instead of generating one HTML page, you're generating lots of XML fragments).