Unlocked iPhones Finally Available in the U.S.
Previously, U.S. users who wanted to have an unlocked iPhone would have to jailbreak and unlock (two separate processes) their iPhones. Those processes, while deemed legal last year, void iPhone warranties.
Earlier reports were that unlocked iPhones would be available in the U.S. beginning on Wednesday. Essentially, this is (sort of) the T-Mobile iPhone that folks asked about when AT&T first announced plans to acquire rival GSM carrier T-Mobile. The (as expected) higher price reflects the fact that since these are unlocked, there's no carrier subsidy or two year contract.
Despite the fact that iPhone users can now purchase official, unlocked phones, there are still two problems. For one, because of the differences between AT&T and T-Mobile 3G frequencies, T-Mobile users will only be able to use the iPhone 4 on much slower EDGE. Additionally, AT&T is still unable , at least for now, to unlock existing iPhones.
[Realistically, this is another blow to AT&T, which lost U.S. iPhone exclusivity earlier in 2011, when Verizon began selling iPhones. As such, unlocking existing iPhones could hardly be seen as a top priority for AT&T.]

Buyers of unlocked iPhones should expect a few days of wait time; black versions have a 1-3 day delay before shipping; white models take 3-5 days to ship (at least, for now).
Users of unlocked iPhones will need a micro SIM, not a SIM, from their carrier of choice. Regular SIMs can be cut down to fit a micro SIM slot, using a kit (or even, just being careful and following online instructions).