The Google AdSense Team recently announced that AdSense for Feeds will be retired on December 3, 2012.

On that day, users who had previously enabled this feature will no longer see Ads served to their RSS feeds or Feed Units in their My Ads Tab.

Even after the service has retired users will still be able to access their performance reports for previously run Ad Units.

Source: https://support.google.com/adsense/bin/answer.py?hl=en&answer=2777193