There have been many discussions about RSS ads lately (including my own), but so far most of these debates haven't addressed ads in aggregated feeds.
Here's the deal: I believe that as more people rely on RSS for information, we'll see the problem of feed overload come to the forefront. The way this usually works is that you start using RSS like email, subscribing to a few feeds and reading everything that comes into your RSS reader. As time goes on and you continue subscribing to more feeds, you eventually reach the point where there's too much information coming in (and too little of it that really interests you).
While there's a lot that RSS readers can - and will - do to address this problem, I believe we'll also rely on human "editors" to find stuff for us. These are people like Scoble, who live in their aggregators and use their blogs to share the links that interest them. Rather than subscribe to a dozen feeds about a particular topic, we might subscribe to a single feed offered by someone who is an editor for that particular topic - for example, rather than subscribe to several PR feeds, we might just subscribe to Steve Rubel's feed and let him point out the interesting links. These editors are the ones poised to benefit the most from RSS ad revenue.
Combined with that, we'll also rely on RSS search engines like Feedster, BlogDigger, Technorati and PubSub to find items that interest us. Forget subscribing to a hundred feeds that might have an item of interest every now and then - just subscribe to an aggregated search feed which looks for your keywords.
Few of us would be willing to pay for RSS search results, so the RSS search engines will most likely rely heavily on ad revenue. This to me is where RSS ads get tricky, and it's something we should think about. If you use Google AdSense in your feed, what happens when items from your feed end up in the search results of an aggregate feed? Should your ads stay in place? Is it okay for the search engine to strip your ads and insert their own? How about if they insert their ads in addition to yours?
What if a search engine offers ad-free feeds for a nominal fee - is it ethical for them to make money by stripping out your source of revenue?
And if the search engines leave everyone's ads in place, how will the aggregated search feed look in your RSS reader if each item uses a different type of ad service? Will the hodgepodge of ad styles be so distracting that we'll find some other way to locate items of interest?
I'm sure others have thought about these issues far more than I have, so it will be interesting to see where this discussion leads us.