Let me circle this back to ads in RSS feeds. You can be fairly sure that every single person subscribed to your feed is a daily reader and it's not likely random searchers would add your feed. The people reading your feed are using a feed because they don't want to miss a single word you're saying. They're not just fans reading your site, they're more die-hard than that. Who would you subject to advertising, if you had a say in the matter: random visitors or your biggest fans?
I think Matt is overlooking the fact that RSS is beginning to cross over from personal publishing to commercial publishing. News organizations and professional bloggers aren't necessarily subscribed to because die-hard fans don't want to miss a word. Of the few hundred feeds I'm subcribed to, a very small (and shrinking!) percentage are personal subscriptions. The majority of my subscriptions are "industry" related - topical news aggregrators, keywords from Del.icio.us, and single-issue weblogs like Read/Write Web or if:Book feature prominently.
RSS is now my primary approach to many of these sites. A number of them subsist entirely on ad money. While I'm not thrilled to be advertised at in my own feed reader, I do acknowledge that a lot of these high-volume skim-feeds may not ever get a proper visit from me, and therefore need to push their support mechanism out where I can see it. As long as the ads stay plain text and on topic, I'll suffer them.