Google has retired their Adsense referral program. Here is the email they sent:

Thank you for participating in the AdSense Referrals program.
We’re writing to let you know that we will be retiring the AdSense
Referrals program during the last week of August. We appreciate
your patience during this transition and here are some alternative
options to consider:

* Google Affiliate Network: As part of the integration of
DoubleClick, the DoubleClick Performics Affiliate Network will now
operate as the Google Affiliate Network for advertisers targeting
users located in the United States. Similar to the AdSense
Referrals program, the Google Affiliate Network enables publishers
to apply for advertiser programs and get paid based on
advertiser-defined actions instead of clicks or impressions. For
further details, please visit: www.google.com/ads/affiliatenetwork.

* AdSense for content ads: If you have less than three AdSense
for content ad units on a page, you may wish to replace the
referral ad units with standard AFC ad units.

If you currently use referral ads, either to promote Google
products or offerings from AdWords advertisers, AdSense Referrals
code will no longer display ads beginning the last week of August.
We encourage you to take the following steps before the product is
retired:

* Remove the referral code from your site(s): Please take a
moment to remove all referral code from your sites before the last
week of August, so you can continue to effectively monetize your
ad space.

* Run and save all referrals reports on your desktop: Create
and save all reports related to the referrals program on your
desktop, so you continue to have access to your valuable campaign
information