The previous five-per-person, cash is a-okay policy for iPhone sales has been changed to two-per-person, leave your cash at home. This is in order to "discourage unauthorized resellers," according to the Apple spokesperson Natalie Kerris.
Almost a million and a half iPhones have been sold so far and roughly 250,000 have been unlocked so as to work on networks other than AT&T. Some of the aforementioned resellers are thought to be involved with facilitating the unlocking process.
Apple Limits Sale of iPhones: Two Per Person and No Cash [AP/New York Times]