Let me clear a few things up. First off, this technology is far from new. The XMLHttpRequest object has been around since Internet Explorer 5.0. I know that I've personally used this technique as long as 2 years ago. Secondly, AJaX will in no way be responsible for replacing desktop applications. An implementation like GMail or Google Maps is ridculously difficult to implement properly.
The latest round of AJaX enabled applications (Google Maps and GMail) are quite impressive. I'm not going to take anything away from these applications, especially considering that they are running in a web browser. However, what is the user experience when compared to a desktop application such as Thunderbird or Outlook.
For the developer's toolbelt, AJaX is just like anything else. You wouldnt use a brand new screwdriver to drive a nail. A good developer will keep AJaX in their toolbelt and use it when appropriate.
I'm afraid whats going to happen here is that every non-technical manager-type person is going to hear 'AJaX' and start asking the development staff how to leverage this technology. If the requirements call for it, then use it. However, do not make the requirements fit the technology.
All this being said, there are some good resources available on the web, including: