1 million 4S's in the first 24 hours

[GGD]

I think there were two things that greatly helped this.

1) Sprint, first time available for it's customers.
2) Verizon, I suspect that a lot of customers didn't immediately jump on the iPhone 4 when it came to Verizon in Jan, expecting a new version of the phone in June, and then continued to wait when nothing appeared in June.

This was the first US intro of a new iPhone simultaneously available to customers of 3 carriers.