Over on Slate's The Big Money blog, Marion Maneker discusses "Secrets of the Amazon Best-Seller List," with speculation on how the list is derived, and how some authors attempt to game the system.
In short, Maneker quotes author Andy Kessler, who speculates:
"I'm not sure the exact number," Kessler says of the weightings, "but my guess is 40 percent hour, 30 percent day, 20 percent week, and 10 percent month. So if you have a huge spike in sales, you don't completely dislodge books that have been in the top 10 or top 100 for months and months. Though you might pass them for a very fun hour."
(link via Marc-Oliver Frisch)