Chat application AJAX polling
You might also want to look into Comet.
It's used by GTalk, Meebo, and many other chat applications. A few years ago when I was experimenting with it, there weren't very many libraries or details about server architecture to implement it, but it looks like there is a lot more stuff out now.
Have a look at the cometd project for more technical information.
If you don't like the idea of HTTP-polling, you could have a Flash-movie on the chat page that has a constant connection to some deamon on the server, the Flash-movie would then invoke JavaScript functions on the client to update the chat as new messages comes along. (Unless you want a Flash interface for your chat..)