Does C2DM registration id change when a different user logs in?

From the documents it appears that there should be a valid Google-account on the phone before an application signs up and gets its registration id.

What happens when the user signs out of that application, and some new user signs in. Does Google issue a new registration id?

To push messages to a device you need the registartion id from the device. You have to generate this id in your app via registration intent. This registration id is an ID generated from the app, your device and your google account.

Once you have the ID and you are registered for C2DM you can push messages on the device with this id. You will call an URL from google and post the register id through the url. Google then sends the push to the device (Google knows you because you are logged in in your google account on your device).

If you change the Google account you won't get any pushes, because the registration id is also generated with your Google ID.

You have to call for a new registration ID.

Here is a nice tutorial for push.

hope this helps