I think Boing! Boing! has exactly the right approach to ads in RSS feeds. Their feed has the full text of their blog entries, and they’re putting ads in about 1/3rd of those in the RSS feed.
Frankly, I think it’d be perfectly reasonable to put them in each entry. What I can’t stand and unsubscribe from immediately are the folks who have a headline from their blog, followed by an ad, followed by another headline, followed by an ad.