There have been a several of interesting pieces recently on why this idea went from hype to passe to waycoolbuzzwordcompliant in just over 5 years. I'll single out a couple from people who were present at the creation of XmlHttpRequest and have seen it rise, fall, and rise again on the popularity charts: Adam Bosworth helped drive this at MS in the late '90s, and now observes:
Finally, in 1997 or even in 1999 there wasn't a practical way to write these applications to run on all browsers. Today, with work, this is doable.
Kurt Cagle wrote an article about XmlHttpRequest in 2000 and recently discovered " I'm so bleeding edge I'm surprised I haven't died from blood loss" now that AJAX has more buzz than a hyped-up hornet. He attributes the buzz to the fact that: 'Opera, Safari and Mozilla Firefox all adopted the XMLHttpRequest object as an inbuilt component with KDE's Konqueror recently joining the ranks. This was a critical turning point, as it meant that at least desktop clients now have the capability of communicating with the server "out-of-band".'
I guess the observation that everything old becomes new again in the world of fashion is hardly novel, but those who live on the bleeding edge need to recall it now and then.