Philipp Lenssen reports that some AdSense publishers were invited to partake in AdSense ads that pay based on action, like an affiliate network. These CPA (cost per action) invites were first detailed at SeekingAlpha.com and the ads do not compete with the contextual network, they run on a separate network named the Content Referral network. Jennifer may have more details later on this news, but in the meantime you can read my coverage from the forum buzz.
Update from Jennifer Slegg: This beta test is on an invite-only basis, and is implemented through a separate ad unit. However, stats are not available real-time and must be emailed to participating publishers, meaning it will be difficult for publishers to gauge how effectively Content Referral ads are performing.
Google also confirmed the beta test: "We're always looking for new ways to provide effective and useful features to advertisers, publishers, and users. As part of these efforts we are currently testing a cost per action pricing model to give advertisers more flexibility and provide publishers another way to earn revenue through AdSense. We're pleased with how the test is progressing and will continue to gather feedback from advertisers and publishers."
For a much more detailed look into this new program, JenSense dives into all the particulars of Content Referrals.