MarketingSherpa is offering a case study on the effectiveness of RSS advertising and podcasting sponsorship. It's worth a look.
One of the things this case study addresses is the low number of sites that are accepting ads in their RSS feeds right now. However, this case study focuses on individual blogs, that may not have really large syndication. Larger RSS campaigns can be run at places like Feedster. We know Sun Microsystems has been running RSS ad campaigns, and they plan on expanding these efforts.