This seems to be a logical move by Google considering they’ve already got Urchin (now Google Analytics) on board. Besides, Google is looking eyeballs from popular sites running RSS feeds and Feedburner’s Ad Network may seem to be the next step in covering all its bases. I know RSS readers are smarter than your average visitor so a CPM or CPA model might work.
Actually, you’ve not yet noticed, there’s AdSense for feeds now available thru Feedburner. However, one will need to apply for that in order to employ it. I’ve applied for it for months now and haven’t heard from them yet.
On another note, Google’s BlogSearch has been reported to include into account the feed subscription count of a blog. By tapping into the database of Feedburner, Google will have access to hundreds of thousands of blog’s RSS usage and feed count for a better blog search algo.
Then, there’s also the podcasts — audio search, possibly? That $100 Million will be well worth it.