As noted by Dave and Sam, InfoWorld announced today that they will introduce ads in their RSS feed. Before I comment, I should disclose that I write for InfoWorld and am on the masthead as a contributing editor. Believe it or not, I thought about this before I knew InfoWorld was going to do it and my thoughts haven't changed.
I view this pragmatically: I'd rather see ads in RSS than not see the RSS at all. I'd rather have InfoWorld and other news sources, including the NY Times show up in my aggregator than have to go to each and every site and search for what interests me. These guys aren't going to just give this content away and as RSS gets more and more popular they might choose not to play. That would be a shame, both for me personally and for RSS as a technology.
One of the words frequently associated with RSS is "syndication." Syndication means that you redistribute content that was created for one venue to others. In the case of most blogs, that is done without restriction or fee. Grab my RSS and go to town. In most other media, syndication comes for a price. If you want to syndicate InfoWorld's content, the price is to look at their ads. Some sites, like Slashdot.org will let you see their site ad free for a fee. Maybe their RSS feed is next?