I recently say another analysis showing that bidding for the number 3 spot is the best use of money. Brandt Dainow, CEO of ThinkMetrics, after reviewed three years worth(1500 keyword sets!) of click-through trends and found the click-through rates (CTR) were virtually identical between ad positions 1-3 in his campaigns.
Others disagree, but in our testing (limited, not 1500 keyword sets!) we have found the same result: positions 3 and 4 are your best use of money. (In most cases. Your mileage may vary. Other standard disclaimers here. Banned where prohibited.)