According to a Piper Jaffray survey (reported in Fortune), three-fourths of iPhone 4 first day buyers were existing iPhone owners who were upgrading: 

• 77% of the iPhone 4 buyers were upgrading from a previous version vs 56% in 2009
• 16% of buyers were switchers to AT&T from another carrier, down from the previous year
• 28% of iPhone 4 buyers were also iPad owners

Various estimates have put first day sales at between 1 million and 1.5 million iPhone 4 units. This is good news (of a sort) for Apple but not as good for carrier AT&T, which is subsiding all the phones.

Obviously the phone is hugely popular but it isn't winning as many coverts as past iPhones. Most of the would-be switchers have already moved over. And increasingly competitive Android devices make people less likely to switch. 

Yet there's still demand among non-AT&T customers for the iPhone that Apple is failing to capture. Previous surveys have indicated substantial demand for the iPhone, for example, among Verizon customers:

If the data in these charts is representative of the broader US market, they represent millions of iPhone units that Apple is not selling -- and that are going to Android or other handsets -- because of AT&T exclusivity.