Ad Shortage?

Two different sites I've visited today, one high profile (YouTube) and the other fairly low profile (sorry Rory) have been showing Google's Public Service Announcement Ads.

Google needs to have an AdSense ad to sell into every AdWords ad spot that's created on the Web. If they don't have an ad to put on your site, they instead show these Public Service Announcement type ads, to fill the space.

Usually you get the PSA ads when your site is new and Google hasn't figured out what kind of ads to post there yet, but these were established sites.

Very strange. I wonder if Google is experiencing a bit of an ad shortage?