It's not a brain as was previously theorized.
This is the main piece of the Revolution controller. It's wireless, and can also detect movements of the controller, such as it can be used to 'look' in FPS games, the way a mouse is used on a PC.
The controller also has a port for attachments on the bottom, and one such attachment is an analog stick that you hold in your other hand for a "nunchuku-style controller".
Read the article on Nintendo.com.
Edit: I just saw Vegeta's thread, sorry about that.