The Black App splits its application into a number of different tiers, each limited to a certain number of registrations. It’s the same concept that credit card companies use, but on the web.
I signed up for the black account and I don’t even have an iphone, but it sure made me feel warm and fuzzy inside when I got the confirmation email. The only flaw I can think of is that the system will likely get you a lot of one-time registrations. Seth Goodin would probably hate this thing.