As you are probably aware, when browsing the web-based PlayStore, it is actually possible to install an app right-away. Upon pressing the "Install" button (like the one in the image - taken from the app page), I immediately receive a notification on my phone telling me that the app is being downloaded.
I would be very interested to know how Google implemented this feature. I suppose being logged to my Google account on both the PC and the phone helps, but still - is the phone continuously polling some network resource to see whether it should start downloading an app? Or is the notification somehow pushed to the phone (I cannot imagine how this would be done)?