Hobbs, I didn't say I haven't been affected. My average EPC and eCPM were lower in February than they were a year earlier. That doesn't mean Google is cheating me, however, just as a drop in the value of my equity mutual-fund shares doesn't mean the fund managers are stealing from my retirement account.
I personally think that many AdSense publishers have been hurt by dilution as "content network" inventory has grown exponentially. The introduction of separate bidding for search and content ads in 2006 has almost certainly been a factor, too. In my sector, growth in display advertising may also be hurting AdSense revenues: I used to see AdSense ads for airlines and hotel chains; today, many of those same advertisers are buying display ads through specialty ad networks and rep firms.
BTW, I think dilution may be as big a concern for "content network" advertisers as for publishers. A couple of years ago, an advertiser for, say, Elbonian luxury raft cruises could buy AdSense ads and be fairly certain that the ads would run mostly on editorial or travel-agency pages that reached readers who were researching purchases. That's no longer the case, thanks to the exponential growth in MFAs, "Web 2.0" sites with keyword-driven, computer-generated pages on every conceivable topic, etc. If I were a hotel owner in Rome or a cruise travel agent in Poughkeepsie, I'd probably be leery of CPC ads on the "content network" myself, if only I'd be afraid that my money was disappearing into a black hole. (With luck, the introduction of site-targeted contextual ads in a few months will bring in or bring back cautious or skeptical advertisers.)