Over the last week or two some Feedburner Ad Network publishers have been transitioned over to the new AdSense Feed Advertising system and over the weekend AdSense for Feeds has gone live for everyone. You should now see them in your AdSense setup tab.
AdSense for feeds is similar to most AdSense ads in that the ads served in your feeds are a mixture of CPM and CPC ads.
Publishers setting up AdSense for feeds have a number of options when it comes to ad formats (text alone, text and image and just image ads), design (colors) and how often ads are displayed (you can have ads appear every 1,2,3 posts or only on posts over a certain amount of words).
If you’re a Feedburner Ad Network publisher you now need to set up AdSense for feeds to keep monetizing your feeds.
For more information see the AdSense Blog announcement.