Interesting...you think of all the A/B metrics that Google tests, this would be among one of the most obvious standouts. The percentage of users who click on lower-ranked results is reportedly a big factor in the decision of how results are laid out and paginated (e.g. if the 12th result in Google SERP had a relatively similar amount of "usefulness" as the 10th, it's likely that Google would have the search pages show 12 results)...you'd think a statistical drop in first-result clicks would trigger an analytics alarm.