Joel gets it right, well...mostly.
Joel Spolsky talks about Google is really making cool stuff with the XmlHttpRequest technique. He says:
"The internal cool thing is that it's one of the first prominent uses I've seen of the IFRAME XmlHttpRequest technique of going back to the web server for more data while the user interacts with a page. This has been possible for a long long time, but web developers have been mostly ignoring it."
Joel doesn't mention it but, Googles' GMail also make extensive use of the XmlHttpRequest technique to do data retrieval. Read this. (scroll down towards the bottom). So does OddPost, who, I believe, was the first to really perfect the XmlHttpRequest technique.
Also, I really hope more web developers start trying to use this technique. It will definitely require that they learn more JavaScript but, that's a good thing. Joel continues:
"The latency of web UIs, in which everything you do is a slow round-trip that requires completely refetching and rebuilding the web page, is one of the reason web UIs feel so clunky compared to native GUIs. Google is very publicly raising the bar on the quality of interfaces that people will expect from web pages."
So, Joel, when are you moving FogBugz to use XmlHttpRequest? :-)