As far as I know, C2DM is per app per device thing..but my question is ,what is the use of device id and where do we use device id in C2DM?
Device Id term is used in iPhone and Registration ID is the term which we use in Android.
Registration ID
An ID issued by the C2DM servers to the Android application that allows it to receive messages. Once the application has the registration ID, it sends it to the third-party application server, which uses it to identify each device that has registered to receive messages for a given application. In other words, a registration ID is tied to a particular application running on a particular device.
Note: For detail description follow the link.