OK, how do push notifications work?
I found a nice tutorial: http://lessons.runrev.com/m/4069/l/59312
I get the concept that the device needs to register with the server but does the app essentially hold an AJAX connection to server or does the server somehow plug into the mobile phone network… ?
There are three essential components when talking about push notifications:
Your server is the sender. It sends the push notification to the providers server, which in turn sends it to the device. Your device, therefore, is the receiver. (Concluding that you don’t have to “plug into the mobile phone network”, this is all done by your provider – Google in this case)
Within your mobile application you just register an eventhandler which is fired when you receive a push notification.
Google called this C2DM (Cloud to Device Messaging), now GCM (Google Cloud Messaging) – you can get an introduction inclusive of an architectural overview and demos here at android.com.