Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_0 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/3.1.1 Mobile/7A259g Safari/525.20)
Its determined by number of downloads. Apple doesn't decide the top apps, the people downloading do. Top paid apps are a few thousand a day I'd guess? Wasn't iFart selling about 13,000 a day at one point? Top free apps are 100,000 or more.