This is not a new approach since XmlHttpRequest has been around for some time. What is new is that (a) it is being widely used by Google, and (b) a new name was coined for it recently.
Google initially used XmlHttpRequest in Google Suggest. Just start typing slowly, and you will see that Google will query a huge list of words in real time and give you the matching words starting with what you typed. Yahoo Mail has also been using this for its email address lookup.
For a technical discussion on how this is done, someone dissected what is being done, and wrote a useful report.
What is also interesting is that Google must keep all these words in an in-memory cache. There is no room for disk access here. The results are displayed instantly.
Ajax is born