C2DM Implementation Checklist Step by Step

Android C2DM helps your app server push message to your app. It's a mechanism that the server ask the mobile app to fetch new data from server and update itself. C2DM is still in beta version, or Labs version. To use C2DM with your Android applications, you must first signup to request access.

The official doc has a lot of content.
To make it clear, I make the following checklist:

C2DM: Get ClientLogin Authentication Token

To send C2DM messages, you should create a new google account as an app-based sender ID and get the ClientLogin Authentication Token.
The most convenient way is using Curl:

curl https://www.google.com/accounts/ClientLogin -d Email=$1 -d Passwd=$2 -d accountType=GOOGLE -d source=companyName-applicationName-versionID -d service=ac2dm

Android Push Notification Service and C2DM

Apple has support push notification since iOS 3.0
But what about Android??
Android can do this using a long-polling service or keep connected to server.
There are implementations of Android push notification.

1. Cloud to Device Messaging (C2DM) official service by Google
2. AndriodPN at http://sourceforge.net/projects/androidpn/files/
3. Urban Airship Service
4. IBM's MQTT