From Wired:
Oneupweb found that the first month a site appeared on the second or third page of Google results, traffic increased five times from the previous month, and in the second month, traffic was nine times greater. The number of unique visitors tripled when a company moved up from page two to page one, and in the second month doubled again to more than six times the traffic it received before it broke the top 10. More importantly, Oneupweb discovered a correlating impact on sales: 42 percent more the first month, and nearly double the second month.Wow.