For quite some time I ran affiliate marketing ads in my feed using the PostPost plugin I developed for WordPress and Commission Junction advertisements. It wasn't really a source of any significant revenue, though, perhaps a few dollars a month.
This week, I applied with Google Adsense to convert my feed from Feedburner to Google. The process to do this is still manual, but fear not, you won't lose a single subscriber in the process. Both addresses continue to publish the feed, but putting the feed on Google has monetary benefits since it will provide you access to Google Adsense for feeds.
Simply login to your Adsense account and follow the directions to manually submit your feed for conversion. It took about a week for mine to get converted. Google is promising that this will be an automated process in the future.
It's still not perfect, though! Take a look at the above ad from a radical political organization who paired up with my note about Joe Biden. The referenced article was actually a pretty scary look at Biden's 40 year history of voting with big brother and leaving the little guy out to hang.