Indeed, yet MANY seem to be ignorant of it. Facebook, Twitter, Google Voice, TweetDeck and many others that would benefit from the easy push notifications and the lessened battery use seem unwilling to implement it. Google Voice is the stunner in my opinion.
I thought Google Voice has been doing this for nearly a year and a half. Maybe not using the same C2DM framework you and I can use, but it certainly seems like they are using a push protocol like Google Talk & Gmail on Android use and are not polling all the time.
edit: it also looks like Facebook has been using C2DM since an update in December of last year:
Neither use C2DM though, they use their own solutions and they frankly don't work well. My email is pushed to me within 120 seconds of it appearing in my inbox. I never get Facebook push notifications (I just simply never have, and it is configured properly), the Messages are terribly inconsistent (they appear in the app, the Messenger app, and sometimes my desktop will grab a message and I'm completely unable to access it from either Facebook app). Additionally, Google Voice will received messages, 10, 20 minutes "late", with no explanation or reason.
I have some friends that applied 4-5 months ago; after getting no response, they retried a little over a month ago, and supposedly got accepted in a few hours. Maybe they are not bothering to approximate a FIFO queue.
I applied two weeks ago and was accepted in a few hours. It took 3 days to activate my production quota after I emailed to request its activation. I am still testing a few more features before I fully launch the new version of my program but I have to say A2DM is really easy to use.
