Following the purchase by Google of Feedburner, one of the only other players in RSS advertising is Pheedo. This currently counts Transcosmos as an investor. Of course, here in the UK Fedafi -which at one point during the last year was put up for sale – should feel their future a little more secure now Google is effectively putting its stamp of approval on RSS adverts.
One other point: Publishers tend to hate Google because it wrestles ad inventory from them for peanuts. What will they now think about Google having a lock on ads in their feeds? Many won't like it – but it's an early market for RSS ads and any extra revenue might be welcome. Especially for smaller publishers, like bloggers. We shall see…