We reported a couple days ago that Apple might start selling factory unlocked iPhones in the US, just like it has been in many countries for over a year. Obviously, there would be an additional cost to that.
Looking at what the iPhone 4 sells for in Canada, both locked and factory unlocked, I assume that if the iPhone 4 is available, unlocked, in the US tomorrow, it will probably sell for $649 for the 16GB version or $749 for the 32GB...