It's been in beta for 2+ years. Here's an example: http://feeds.feedburner.com/uncrate Maybe Google was waiting for the Feedburner acquisition. Dunno.
trichnosis, adsense RSS is currently running live only for a few publishers. So its possible but not available. If you want ads in feeds, feedburner is the best option. Now google owns feedburner so its better than the best
maybe you would like to read further https://www.google.com/adsense/support/bin/topic.py?topic=957 https://www.google.com/adsense/support/bin/answer.py?answer=20012&topic=957 hope that helps some queries