We reported in the past of some publishers noticing AdWords in the referrals section of their AdSense portal. But now it is officially open to everyone with an AdSense account.
Here is a typical ad:
Forum discussion at WebmasterWorld and DigitalPoint Forums
FAQs on referrals here.

Comments:
adsense master
04/06/2006 10:23 am
On their blog they say that you will earn 20$ at every 100$ spent by the advertiser, but on the AdSense control panel they say you will earn only 20$ at the first spent. This is confusing.