Google's 2009 internal research indicates this isn't true:

http://adwords.blogspot.com.au/2009/08/conversion-rates-dont...

"...on average, there is very little variation in conversion rates by position for the same ad. For example, for pages where 11 ads are shown the conversion rate varies by less than 5% across positions. In other words, an ad that had a 1.0% conversion rate in the best position, would have about a 0.95% conversion rate in the worst position, on average."