The iPhone 4 is now available unlocked.

Apple has officially started selling an unlocked iPhone 4.

The company's online store now lists both a 16GB and 32GB unlocked version of the smartphone. Customers can choose between a black or white model. The company's 16GB version retails for $649, while the 32GB option is available for $749.

Apple currently sells locked versions of the iPhone for $199 and $299 for the 16GB and 32GB models, respectively.

"If you don't want a multiyear service contract or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone 4 is the best choice," Apple writes on the device's product page. "It arrives without a micro-SIM card, so you'll need an active micro-SIM card from any supported GSM carrier worldwide."

That last statement is perhaps most important to potential customers. Apple's unlocked smartphones will only be capable of running on AT&T and T-Mobile networks in the U.S. The company said it will also work around the world on "supported GSM carriers."

Unlocking the iPhone has long been the practice of some iPhone owners. Over the years, several unlocking tools have been made available to people who wanted to break from the grasps of AT&T and try out T-Mobile in the U.S. Last August, for instance, the iPhone Dev Team released an unlock solution for the iPhone 4, called "ultrasn0w," that allowed users who had jailbroken their iPhone 4, iPhone 3GS, and iPhone 3G to bring the device to another carrier.

But those who wanted more official means of unlocking the smartphone have been forced to wait it out in the U.S. while other consumers around the world had their pick of Apple's unlocked devices. Last year, Apple started selling unlocked iPhone 4 units to customers in Canada after it offered the same deal to U.K. customers.

Rumors of an unlocked iPhone 4 hitting the U.S. cropped up over the weekend when Twitter user @chronicwire started divulging details on the devices. The person correctly claimed that Apple would be selling the 16GB and 32GB black and white iPhone 4 models.

But exactly how long U.S. customers will be able to enjoy the unlocked option remains to be seen.

Earlier this year, AT&T announced its plans to acquire T-Mobile USA from Deutsche Telekom for $39 billion. If that deal is approved by regulators in the coming months, there will be just one GSM carrier in the U.S., and the need for an unlocked iPhone 4 will be diminished.

However, it won't be eliminated. As Apple points out numerous times on its unlocked-iPhone 4 page, the devices are especially useful for global travelers who want to quickly switch carriers while on the go. Around the world, GSM is the overwhelmingly dominant technology for mobile communications.

"Buying an unlocked iPhone 4 allows you to choose your own GSM carrier, change carriers at any time, and even use multiple carriers if you travel frequently," Apple points out on its unlocked-iPhone 4 page.

Apple's newly unlocked iPhones will ship in one to three business days.

Updated at 4:48 a.m. PT and at 5:22 a.m. PT: Added more details on the unlocked iPhones.