Latest Google AdSense Blog post is titled “Introducing multiple custom channels”.

An AdSense Publisher Support, Arlene Lee, says:

“As of today, it is now possible to assign multiple custom channels to a single ad unit. This feature enables you to track your ad performance with greater flexibility and view more granular information. When generating your ad code, you’ll be able to add up to 5 custom channels to a specific instance of ad code”. 

You can read the whole article at:

http://adsense.blogspot.com/2006/10/introducing-multiple-custom-channels.html