I've got a GAE app running just to test this out. An example feed would be http://pushfeedtest.appspot.com/feed/b<http://pushfeedtest.appspot.com/feed/lol2> lah 'blah' can be any string (different feeds).
The hub is told that there are changes every 1 minute (alternatively, http://pushfeedtest.appspot.com/p<http://pushfeedtest.appspot.com/feed/lol2>ump to force it). For some reason the subscriber I found at http://bitbucket.org/pstatic/pubsubhubbub/wiki/Home works fine, but my own, push-bot.appspot.com and even Google Reader don't work until the bitbucket one subscribes, and then they all start working.
