The evidence is mounting, while it is true that sites in the #1 spot do get more clicks, do they get more sales? Typically shoppers (people educating themselves and getting ready to buy) will do a little research before they turn into actual buyers.
Shoppers will click the top ad, buyers usually click several ads, and more times than not, they end up realizing all the products and pricing is about the same, and where are they when they come to this conclusion. They are usually on the second or third site down the list.
So what does this mean to you? The best Pay per Click ROI can usually be found somewhere between spot #2 and spot #4.