This is a Slashdot thread worth following: AJAX vs. Server Load
On the heels of that thread, Jakob Nielsen has published a piece "Why Ajax Sucks (Most of the Time)" He talks about how 22% of the users cannot use/benefit from Ajax. He looks at how it hindered search engine optimization and its hard to print. Boo-hoo.
In 1997, it was cool to do a website in white text on black. Very edgy and when you printed it, out came page after page of white text on white paper. Ajax is a fringe tool that I think will push its way into the mainstream. When I have time over the Christmas break, I will publish a piece on "Discreet Introduction" and show that whether you love Ajax or loathe it, there are many tried and true strategies for getting new content to the page without hitting the "refresh" button.