If you are looking to create an instant messenger for Android, this code should get you started somewhere.
Excerpt from the source :
This is a simple IM application runs on Android, application makes http request to a server, implemented in php and mysql, to authenticate, to register and to get the other friends' status and data, then it communicates with other applications in other devices by socket interface.
EDIT : Just found this! Maybe it's not related to WhatsApp. But you can use the source to understand how chat applications are programmed.
There is a website called Scringo. These awesome people provide their own SDK which you can integrate in your existing application to exploit cool features like radaring, chatting, feedback, etc. So if you are looking to integrate chat in application, you could just use their SDK. And did I say the best part? It's free!
*UPDATE : * Scringo services will be closed down on 15 February, 2015.