Essentially Google wants publishers to use refferal units other than the Adsense referrals. They claim that “After experimenting with this program over the past year, we’ve concluded that there are other products that are of higher value than this program to publishers”
Here is an except from the original mail from Google to give you a better idea.
If you’re currently displaying referral units on your site directing users to sign up for AdSense, read on below for details about what to expect in the coming weeks.
In early January, the option to add referral units directing users to the AdSense product will no longer appear in your account. You will continue to accrue earnings for all existing referrals yet to generate $100 until late January, at which point the program will be fully retired. Existing referral units will continue to appear on your pages.
By the end of January, you should remove all referral units directing users to AdSense from your pages. Referral units that you do not remove will continue to be displayed on your pages as normal, but conversions will no longer be recorded. We suggest you replace the AdSense referrals with referrals to another product or service or an additional ad unit.
I am unable to see the Adsense refferal units in my account at this point, so out goes the Adsense refferal. Never did any good for me anyways !