Apple began selling so-called “unlocked” iPhone 4’s this week via its U.S. online store. “Unlocked,” in this case, means that the phone isn’t partial to one carrier or another—any supported GSM wireless carrier can be the one you use. Of course, in the U.S., that doesn’t mean a whole lot: an unlocked iPhone 4 will only work over the AT&T 3G network. But it’s a significant perk when traveling abroad. While in another country, you could pop out the micro-SIM card you have and replace it with a local one. To live unlocked also means that you don’t have to sign on to a multi-year service contract, if you’re commitment averse.

If terms like “micro-SIM” and “GSM” are new to you, Apple has a refresher on the page where they’re hawking the new phones. GSM stands for “Global System for Mobile Communications,” and is the most common industry standard for wireless communications. AT&T uses GSM; the Verizon iPhone, though, roams on a different kind of network called CDMA. And a micro-SIM card is the small memory card inside your cell phone that carries information related to your wireless account. Though iPads also use micro-SIM cards, notes the Apple FAQ, you can’t interchange those with the micro-SIM card in your iPhone 4.

The specs on the phone are the same as in any iPhone 4: you’ve still got FaceTime, HD recording, a 5-megapixel camera, and so on. You can get the unlocked iPhone in a 16 GB version (for $649) or a 32 GB one (for $749). Those prices are a significant hike, it should be noted, over the AT&T and Verizon versions; with either of those carriers, the 16 GB starts at $199 and the 32 GB at $299. The unlocked versions, like the locked versions, are available in white as well as black.

While an unlocked iPhone 4 is a real boon to some frequent travelers or those with an allergy to multi-year service contracts, we have to admit that what would be really interesting would be if Apple were offering not just an unlocked phone, but a so-called “jailbroken” one. Jailbroken iPhones can use any carrier, hardware permitting (T-Mobile, for instance); furthermore, they offer access to a range of third-party apps that didn’t have to go through Apple’s rigorous and slightly mysterious app store approval process. Back in 2007, Slate contributor Tim Wu joined the many who had tried it, and declared it “legal, ethical, and just plain fun.”

Is Apple’s offering of an unlocked phone a pointer to the day when it offers a jailbroken one? Unlikely. Apple has held fiercely to its anti-jailbreaking stance (which makes business sense, of course, since any fees paid outside of its sanctioned app store don’t make their way to Apple). Apple, for its part, has said that jailbreaking phones can be unsafe, leading to “instability, disruption of service, and other issues.” It has further said that jailbreakers risk voiding their warranties.

So be careful: while getting an unlocked phone costs you a few hundred extra buck, getting (or making) a jailbroken one could cost you any future support from Apple.