I don't know if it's true or not but some have noticed Google putting fewer ads on their search results.  According to Robert Scoble:

"…do a Google search for: San Francisco Sushi.

You’ll find two ads along the top, and three along the side. In a second search I did, the ads on top disappeared and there were only five ads along the side.

There used to be more, the Google employee told me. But, he said, Google has done a lot of research with users and found that fewer ads mean less revenue SHORT TERM. But long term the advertising revenue actually goes up. Why? They found their users started trusting the advertising more and were more likely to click on ads."

I don't think we need to look for esoteric ideas, maybe people are advertising a little less on AdWords because it's getting more expensive and the results are usually not that good.

Conversionrater also has an analysis which adds PPA as one possibility for a change in the number of ads but he does not buy the idea Google is doing this to get searchers to trust it a little more. Sounds like there's more to this story we don't know yet.