Following on the heels of a weekend rumor, Apple’s U.S. online store opened its virtual doors this morning with a new entry to its lineup -- the same old iPhone 4 you already know and love, available in black or white, 16GB or 32GB, but with one very different change.

Apple has surprised many skeptics on Tuesday morning by adding an unlocked GSM iPhone 4 to its lineup in the United States. While unlocked phones are quite normal in Europe and other countries where customers generally pick the prepaid service they want to use and then change phones (including the iPhone) at a whim, the move is a first for Apple in this country.

So what’s the difference between a locked and unlocked iPhone 4? First of all, price: A 16GB iPhone 4 locked to AT&T will only cost you $199 (plus signing in blood with the carrier for two years), while the unlocked model -- which you’re free to use anywhere you’d like -- will set you back $649. Likewise, the 32GB model locked costs $299 but runs $749 unlocked.

“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’s product description explains. “It arrives without a micro-SIM card, so you’ll need an active micro-SIM card from any supported GSM carrier worldwide.”

Apple’s latest move gives the jailbreak community one less reason to exist, since buying a locked iPhone and using hackery to free it has historically been a key reason to do so. With an unlocked handset, official tethering plans and the forthcoming iOS 5, it seems there are suddenly a lot less reasons to jailbreak.

While Apple may not sell as many unlocked handsets here in the U.S. -- especially since they can only be used with AT&T anyway -- the new offering gives many customers the choice of skipping a renewal of their contract for another two years, as well as the freedom to take the device overseas and use a local SIM card for service rather than being tied to AT&T’s oppressive international roaming rates.

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