Some of you may find my post on implementing a Hubbub subscriber on App Engine of interest. One use-case that I emphasise, that I don't think has received as much attention as it should, is that you can use PSHB to make consuming regular feeds a lot less onerous for apps.
http://blog.notdot.net/2010/02/Consuming-RSS-feeds-with-PubSubHubbub -- Nick Johnson, Developer Programs Engineer, App Engine Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration Number: 368047
