Yahoo's Overture takes the first steps towards advertising in RSS feeds.
Sigh. I really don't think advertising in RSS is a good idea. I'd rather see excerpts only in RSS feeds and advertising on websites when people clickthrough to read the full article.
But it seems that advertising in RSS is inevitable. I hope it's well targeted and unobtrusive, like Google AdWords, not untargeted and obnoxious, like popup advertising.
Update:Dave Winer comments on advertising in RSS, arguing that excerpt-only feeds are already advertisements (for the site where you have to go to read the full article) and that bloggers should "think carefully about how much advertising you think your readers can endure."
Exactly right. Heavy advertising may generate more short term revenue, but only at a long-term loss in revenue from driving away readers. It's a careful balance. The best advertising is unobnoxious, relevant, and useful.