Rand Wacker wrote

"So do all these "AJAX" developers just assume we're living in a one-browser world now...Even Google with their hordes of developers can only safely support a handful of common browsers."

We're definitely not living in a one-browser world. I wish incompatibilities would have gone away. But with careful design approaches support for IE 5.5+, Firefox, and Safari can serve millions of people (currently the vast majority) very well. It isn't easy, though. The bigger question is how to craft javascript-heavy applications and make them accessible and degrade nicely (say, via unobstrusive DHTML practices).

More on-topic: The dissonance between the browser usage metaphors and those interface metaphors commonly used in native apps needs more careful study. I'm discovering that adoption of Gmail and like-designed apps seems to be playing a part in altering expectations for the usage of Back, Forward, and Refresh and this may only be making the expectation waters brackish rather than clearer. (I'm fascinated by this - I would love to know the expectation model users have of My Yahoo!'s richer elements!)

Despite the burdens and risks, this kind of development can result in much better experiences for people - say, in the case of dragging out-of-visual-bounds contextual information without losing application context.

I would add, though, that I think use of non-URI-request-based interactions (or inline-POST interactions) should be carefully considered case-by-case. An Ajax approach wouldn't be useful or beneficial in every web application.

Also...I *wish* we had hordes of developers! (I'm an "AJAX" developer for Google.)