I received a letter from Google today:

Hello,

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.

* 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.

…

Why is this happening?
We’re constantly looking for ways to improve AdSense by developing
and supporting features which drive the best monetization results
for our publishers. Sometimes, this requires retiring existing
features so we can focus our efforts on the ones that will be most
effective in the long term. For this reason, we will be retiring
the AdSense Referrals program.

…

Sincerely.

The Google AdSense Team