Docs are on their way; just waiting for the push to complete.

Docs are here:

http://code.google.com/appengine/docs/python/channel/

http://code.google.com/appengine/docs/java/channel/

The docs haven't been updated yet. Here this years IO video and pdf presentation:

http://www.google.com/events/io/2010/sessions/building-real-...

and here's the sample app (java):

http://code.google.com/p/dance-dance-robot

Live demo: http://dance-dance-robot.appspot.com/

I found short preview coverage at http://bitshaq.com/2010/09/01/sneak-peak-gae-channel-api/

The Java Channel API documentation: http://code.google.com/appengine/docs/java/channel/

Its in the official docs http://code.google.com/appengine/docs/python/channel/