Apple has released their unlocked iPhone 4 in the US officially through their Apple Stores.
You can buy the 16GB iPhone 4 GSM version for $649 in Apple retail stores (sale starting tomorrow) and in online Apple Store right now.
32GB iPhone 4 will sell for $649. This is $50 cheaper than the previous unlocked iPhones Apple has sold (at $699 & $799).
For those who hate getting tied up in a 2 year contract when the smartphone world is changing so rapidly, buying an unlocked iPhone 4 could be a great option. With the early termination fee running up as high as $375, the high initial cost could be justified if you also take into account the 2-year data plan commitment you save yourself from.
Apple has no word on the iPhone 5 release date and releasing the unlocked version could be a signal of an impending refresh to clear out the inventory.

Besides the US customers, I think International buyers stand to gain a lot here too. There are still many countries where Apple has not reached a deal with the cell carriers and folks had to rely on jailbroken/unlocked iPhone 4 sold via ebay to get them.
Now, they can purchase an officially unlocked iPhone 4 via eBay. For eg. 16GB iPhone 4 in India costs around $763 and you could get someone to buy it for $100 cheaper in the US.
AT&T Month-to-month iPhone 4 Plan
AT&T has not come out with official word yet. But, TUAW (a nice FAQ) reports that you will be able to use the pre-paid PAYG cards with the iPhone 4 with no term commitments.
AT&T has confirmed that there is no term commitment for these plans and you may cancel at any time. It’s clearly cheaper to sign up for a $15 monthly data plan than a PAYG data plan — but they probably will not play nice together.
Can I use Unlocked iPhone 4 on T-Mobile?
Short answer is. Yes, you can but you can’t use it in 3G speeds. Only the slower EDGE connection.
This also bodes well for those here in US who want to buy an iPhone right now and not wait for iPhone 5 or iPhone 4S. They can turn around and sell the unlocked version and get a subsidized AT&T plan when it comes out finally in September.
You can buy an unlocked iPhone 4 right now at the online Apple store.

