The iPhone 4S features Siri, a virtual voice-activated assistant not available on other iOS devices. Photo: Jim Merithew/Wired.com

If you’ve been craving an iPhone 4S, but you travel a lot, or don’t want to be tied down by two-year data plan, you can grab your very own unlocked, contract-free model starting today.

One caveat, however: The unlocked model is only available on GSM networks, such as AT&T or global providers, not on CDMA networks like Verizon or Sprint. Apple is careful to note this on the purchase page of the Apple Store’s website.

For T-Mobile users, that means, yes, you can use an iPhone 4S on your network (as owners of unlocked iPhone 4s are already doing). However, you’ll be limited to 2G/EDGE data speeds due to network differences.

Starting in late October, Apple began shipping unlocked models to a number of European and Asian countries. Sprint also initially sold its iPhone 4S with the SIM unlocked, and Verizon would unlock it if you asked very, very nicely. Sprint just started locking its SIMs, but will unlock them for international use with “customers in good standing” in the future. Apple started selling unlocked GSM versions of the iPhone 4 in June this year.

The unlocked iPhone 4S available directly from Apple ships without a micro-SIM card, so you’ll need to buy your own micro-SIM from any global GSM carrier, and insert it in your iPhone yourself. After you’ve got it in your phone, press and hold the power button for a few seconds, and then you’ll be prompted with onscreen instructions to properly set up your iPhone.

If you want to get your hands on an unlocked 4S, it doesn’t come cheap. Pricing starts at $650 for the 16GB model, and increases to $750 for 32GB and $850 for 64GB. Shipping is estimated at one two two weeks if you order from Apple’s web store.

Another caveat of the contract-free version of the 4S: You may be subject to a credit check, and you must be 18 or older to purchase it. But if you’re game, head on over to Apple’s website to grab one.