#1 most useful thing I learned at @d:tech was from Kevin Lee of did-it.com
who's done extensive research on conversion patterns of consumers who click on search engine results. He's noticed that the type of consumer who clicks on the very first result is FAR LESS LIKELY to convert into a customer of that site, than people who click on the second or third ranked results of their search. Why? Apparently it's due to personality types. People who click on the very first thing are more likely to be impulse clickers who didn't bother to read the decription of the site to see if it was right for their needs. People who click on lower-ranked sites are considered clickers, who carefully decided which site was right for them.
This definitely means if you pay for placement on systems such as Overture or Ah-Ha, you should be tracking the conversion on the clicks you're buying based on what rank you purchased. Make sure you're paying for a rank that brings clickers most likely to convert - not impulse clickers.