Comment on Google I/O 2013 Session Overview: From Nothing to Nirvana in Minutes: Cloud Backend for your Android Application – Building Geek Serendipity by Bill Lahti

I too was getting 404 responses in the Android client. When I looked at the App Engine side it was reporting configuration errors. I found that I could get rid of the errors by setting the values for Audience and Client Ids in the Mobile Backend Settings. More information on Stack Overflow (see http://stackoverflow.com/a/17495210/1685670).