Many publishers choose to monetize their RSS feed by showing ads within the feed.  This is a way for them to monetize their subscribers who only read through RSS and never visit their site.
However, your RSS ad choices are slim.  You can choose to go with Google, Yahoo! or Feedburner, but all of them have [...]

Rebecca Lieb, a ClickZ expert, gives an on-target review of online media activity. She also touches on Ads in Feeds:
RSS Feed Advertising
Every Web site and its brother launched an RSS (define) reader this year, while tech companies raced to populate those feeds with ads. Google’s working on AdSense for feeds; FeedBurner launched a feed [...]