How AT&T Decides If iPhone 3G Owners Get A Subsidized iPhone 3G S

After AT&T announced that select iPhone 3G owners would be eligible for a fully subsidized iPhone 3G S the Internet let out a collective sigh of relief. We were told that early adopters would get the better deal and that eligibility status would be updated on the 18th. Well the 18th came, and many 3G owners, including those who bought on day one, were disappointed.
So what's the deal here? How exactly does AT&T determine if you get a fully subsidized iPhone 3G S? Well I spent most of this morning trying to figure it out and here's how it works: