title: Do GCM registration id's expire?
body: <p>I know that C2DM registrations expire, and you are supposed to periodically refresh the registration ID. Is this the case with GCM? by looking at the following code on the Android GCM guide (shown below), it seems like you only do it once and don't need to refresh, but I dont see that explicitly written anywhere, so I just wanted to check. </p>
  
  <pre><code>final String regId = GCMRegistrar.getRegistrationId(this);
  if (regId.equals("")) {
    GCMRegistrar.register(this, SENDER_ID);
  } else {
    Log.v(TAG, "Already registered");
  }
  </code></pre>