A WebmasterWorld thread confirms what many people have suspected for a long time. The top advertisement inside a Google AdSense unit is the highest paying ad in that unit.
AdSenseAdvisor has confirmed this:
I can confirm that in the general ad auction, our system will target the highest-paying ads to the ad unit implemented first in your HTML, working down through the auction results to fill in the rest of your ad units in order.
Most people knew this, but it is nice to have official confirmation from a Google representative.
Forum discussion at WebmasterWorld.