Update: The unlocked iPhone 4 actually slipped out a day early, and is now available through the Apple Store. As mentioned below, this is a GSM model only. Also, we've answered many common questions in our unlocked iPhone FAQ.
While we were skeptical about the rumors that came up over the weekend, both MacRumors and 9to5 Mac now confirm that unlocked versions of the iPhone 4 have arrived in Apple's retail outlets and will go on sale starting Wednesday. The 16 GB iPhone 4 will cost US$649, while the 32 GB model will retail for $749. These prices are higher than U.S. buyers might be used to seeing for the iPhone because they're handset-only prices, independent of any carrier contract. Note that this concerns only the GSM model; the CDMA/Verizon model is expected to remain Verizon-only for now.
Like most other mobile phones in the U.S., the iPhone has been locked to a specific carrier since its introduction. Before the Verizon iPhone debuted, AT&T was the sole carrier choice for American iPhone owners unless they were willing to jailbreak for the privilege of using the iPhone on an alternate carrier. The iPhone's carrier lock in the U.S. also means that, unlike international users who can swap out SIMs at will, American iPhone owners who travel internationally must pay exorbitant international roaming fees.
As the GSM model iPhone 4 will now be available without an AT&T contract, users who choose to pay the high initial cost for the iPhone will find themselves with a few advantages -- not being chained to AT&T for two years has to be the biggest one. However, given that the only other major GSM carrier in the States is T-Mobile, whose 3G network is incompatible with the iPhone 4, the ability to swap between national networks at will is not as big of an advantage as it may seem at first. The real winners will be international travelers, who will finally be able to swap SIMs and pay lower fees when they're abroad.
This may be a boon to international buyers, as well. Unlocked versions of the iPhone 4 retail for significantly higher than the above-quoted prices in most countries. For example, a contract-free 32 GB iPhone 4 is available in New Zealand for NZ$1328 (US$1084), significantly more expensive than the U.S. price even after taking GST into account. Buying an iPhone from the U.S. has been out of the question for most international purchasers due to the AT&T carrier lock, but if the iPhone 4 and subsequent models will be available unlocked, I know of at least one person who will be taking full advantage of the lower U.S. pricing when the iPhone 5 comes out.