One of the big limitations of web-based programming is the pull nature of the HTTP protocol. Services like Google Talk, etc. get around this by using a concept that’s now called Comet, which allows the server to “push” content to the client, rather than forcing the client to continually ask for updates.
Since I tend to need this functionality more and more in my Ajax-style apps, I think there should be a web service to make this easier. I’ll keep you posted on my progress.