Via the John Battelle Searchblog

Feedster.com is working with Kanoodle to include ads in it's RSS feeds, according to this eweek.com article.
Feedster currently offers RSS feeds of searches performed on its search engine, so users can be automatically notified when new search results for their queries appear.
Subscribers will be able to have ad-free feeds for
$10 a year according to the article.

RSS is still mostly ad-free and it's only a matter of time before ads become commonplace in the format as more people start using it.
The main argument for it is that a lot of sites make a profit only through their content, so to have that valuable content being distributed without any gain makes no sense from a business angle.

The only attempt I've seen so far to monetize feeds is by CBS Marketwatch.com, though infoworld.com also includes ads.
The CBS feeds send out the ads periodically, so sometimes you get a new item notification only to find that there is no items, only ads. This approach is so akin to email spam that I unsubscribed from the feeds, hopefully the Feedster ads will be less annoying.

More info;
· Feedster.com
· Feedster Preps Paid RSS Links as Ads Expand
· RSS Ads - The Business Model for RSS - Why Ads in RSS?
· Is Ad-Supported RSS the Next Big Thing?