How is GMail Chat able to make AJAX requests without client interaction?
That tech is known as "comet", but also as "server push", "reverse ajax", etc.
It's about pushing data from the server to the browser, keeping an http connection alive. Find more info on it on the wikipedia article (English version).
Also here's a pretty good presentation with Joe Walker from DWR, where he talks about comet.
As you rightfully pointed out, HTTP requires data to be 'pulled' by the client. Gmail can still 'pull' data from the server by using a timer to trigger the HTTP operation instead of requiring the user to click something. So, it may seem to be auto, but it is still client initiated.
Yep Comets is correct. Google Web Toolkit Applications by Ryan Dewsbury explains how to create a Comets based Instant Messenger application in chapter 9.