Apparently, Opera 9 supports Server-Sent Events which you can read about in the WHATWG Web Applications 1.0 specification. Their demo application is a little chat program, which is a very obvious example of the technology.
There’s many times when I’ve wanted to add this to my applications: in Bugtagger I have to ask the service if a new message from a customer has been posted to a bug, every n seconds. It would be far more efficient for the serve to send a message using this technology.
And, since Opera’s so popular on embedded and mobile platforms, if this makes its way to mobile phones you’re going to see some seriously interesting mobile web activity.
Chat and messaging are one thing, but couple this with feeds from news sites, calendars and other web services and any mobile device suddenly becomes a persistent tether to the web. Receiving a message on a phone is very natural, but browsing to services can be very awkward for things that are naturally real-time.