We've recently switched to image ads and noticed tons of Google product ads instead of PSA's. Is Google participating in their own auction and bumping PSA's out at the last minute. We've written extensively about our observations http://buzzdroid.com/google/adsense/adsense-ethics-should-google-advertise-google-products-in-adsense/ (sorry, we're new and don't have live link privilege yet). As experts on adsense, what do you think? Is it ethical for Google to participate in their own auctions?