Google has a great introduction to setting up push notifications using their new system: Google Cloud Messaging. For more info see the getting started guide here:
http://developer.android.com/guide/google/gcm/gs.html
Since you are using google app engine, check out the demo app that uses app engine. Here is the server side code segment for app-engine users:
http://developer.android.com/guide/google/gcm/demo.html#appengine-setup
