In the realm of GSM smartphones, there's a phenomenon known as "SIM unlocked," which is the ability to use any SIM card in a phone, meaning you can easily swap from carrier to carrier. And it's pretty rare in the United States. If you buy a myTouch 3G, it's locked to T-Mobile. Same goes for the Motorola Backflip on AT&T. They're both GSM phones and share radio frequencies, but they're locked to the carriers. (The Nexus One is one of the few SIM unlocked phones available -- erm, once available -- in the U.S.)
In order to use one GSM phone on another's carrier, you'll need an unlock code. And obtaining that code (or cracking it) really isn't new. Been doing it for years on the Windows Mobile side of things, and there are websites that will unlock your phone for a small fee. (Or, if you're in good standing with your carrier, it may give you the unlock code.)
Now let's turn to the AT&T Captivate and the T-Mobile Vibrant. Both Samsung Galaxy S-class phones are of the GSM variety and share radio frequencies. And it turns out that the unlock codes are stored on the phones themselves in a hex file, and the fine folks at XDA Developers -- mad props to dagentooboy, rbnet.it, marcopon, Bowsa2511, RazvanG, chcp0112345 and galaxysguy -- have made it dirt simple to extract your phone's code and SIM unlock the beast. And even better is that you can relock them for warranty reasons if need be.
We've done both of our phones, and they worked just fine (though we're only getting EDGE on the Vibrant, for some reason). It's a pretty big step in the name of openness, since buying unlocked phones (which also would mean unsubsidized -- read: more expensive) is pretty much non-existent in the states.
Hit up the XDA Developers link for all the details, read the instructions, and get to unlocking! [XDA Developers via Engadget]