A WebmasterWorld thread reports that many publishers have started to notice that their Google AdSense ads are not showing up when someone accesses their pages via the Google cache.
For example, a recent Google cache copy of my personal blog does not show the AdSense ad at the top right of my blog. It simply does not render on the page. Typically, it would show to the right of the linkedin logo.
Some theories are that it is related to the Allowed Sites AdSense feature, but I don't use that for my personal blog. Others just think it is a temporary Google bug. Time will tell.
Forum discussion at WebmasterWorld.