The numbers indicate that it's a small shift at best. People switch carriers every 3-4 years on average (usually citing employer discounts, pricing plans, family networks, etc,) meaning that for any given phone, you'd expect 25-33% of purchasers to be new customers. The iPhone is somewhere between 30-40%, meaning that it's probably outperforming the average by 5-10%, which is great but not indicative of a sea change in consumer habits.

It's probably more effective at keeping people on AT&T who would otherwise have left than in bringing new people on.