explorador - 9:12 pm on Apr 14, 2011 (gmt 0)
Hi coach, from my perspective there is a chance something changed on your websites.
Target issue perhaps? this would show a variation on your stats patterns. Yes it could be a change on traffic quality, bringing visitors that are not your target and thus, not clicking or buying, have you checked what happens after the visitors get to your site? a change on target (diff visitors profile) would also mean staying less time on your sites and definitely less page views. Visits would tend to bounce, to go away.
If those patterns haven't changed, there is no signal of people leaving right after getting there, then I would check the htmls, JS, IDS (adsense and affiliates) and even the server itself for htaccess issues or server side includes. A site can be hacked to work for other people changing your ids, so everything would be -ok- but no earnings. Compare your stats (adsense) to the pageviews you have on your regular stats reports.
I mentioned *sites* because even as you mentioned that you are seeing this on sites hosted on different servers, there could be a relation on technology, such as a CMS vulnerability or a door you left open by using the same technique on both sites (like permissions 777 to say something).
I once had a crypted JS injected on my HTML that replaced info on the page of a site, and also had a php problem showing random quotes of iran. Double check.