Do SEM Ads Result In Organic Drop?

New Google Test: AdWords Ads Can Result In 15 Point Drop In Organic Click Through Rates:

There haven't been any big studies on the impact of PPC, AdWords ads, on organic click through rates since Google dropped ads from the right rail in February. But a new small test has one advertiser saying their click through rates on their number one listings drop 15 points when their ads show, that is a CTR of 35% dropping to 20% when their ads show. [...]

He wrote:

We've done some tests where we have toned down on PPC, but for the most part this test has been brought around because we're spending 3x as much on PPC for certain commercial keywords. This has impacted the traffic we get through organic significantly. The most extreme case is where we spend a lot more money on PPC and our SEO CTR for keyword goes from 35% CTR to 20% CTR. It's almost like dropping from position 1 for a term to position 2.

Although this study is very interesting, it does raise a few concerns. First, we are talking about brands that rank #1 for the studied search terms. That is certainly the minority of advertisers. Second, what does it mean for organic listings when competitors advertise above them? There would definitely be some lost traffic in that situation. So the question should revolve around the total net impact of the two scenarios, instead of one ad vs one organic listing.