I first learnt Appengine new Channel API around these days. This is a great API that can be used to create Comet (reverse ajax) Application, or non technically, real time web application. The use cases may be real time stock feed, multi-player web game, and more. It makes Comet application development easier with easy-to-understand API and setup.
I have created an Appengine Channel API Code Demo open source repository on Google Code, and have created a first demo, a very and super simple hero game prototype, with the use of HTML5 Canvas implementation. The user is able to control and move the hero and maybe shooting. The most important thing to showcase is the ability to see another hero (remote user) in your browser screen. Both of you can control it and see their move. For now, you may use two browser windows to see the differents.
I would or may update this code repository later, during the process of my research dugging into Channel API.
I should think more use case that can be realized by using this great Channel API later.