iPad Not Locked
It's worth noting that you can do this with either the AT&T or Verizon Wireless flavor of the new iPad. Both contain a SIM tray accessible from the exterior of the iPad, and neither device is locked. Yes, you heard that right. Unlike the phones these carriers sell, the iPad is not locked, and you can change data carriers at will. In fairness, many of the devices that Verizon Wireless sells for global coverage are also unlocked, and you can install a foreign SIM in them if you wish. AT&T, as far as I can tell, doesn't unlock its phones under normal circumstances.This also means that you're not required to stick with the carrier that provided the initial SIM card. You can, for example, buy an AT&T iPad, pop in a T-Mobile SIM and use it on the T-Mobile network. Right now, that means you'd be stuck with a 2G EDGE connection, but by the end of the year, after T-Mobile deploys the new 4G frequencies it got from AT&T when their merger deal broke up, you'll be able to use the new iPad on T-Mobile's 4G network just by buying a new SIM card from T-Mobile. A source at Apple, speaking on background, confirmed that both versions of the iPad have this capability. Whether that means that you can use a Verizon iPad on T-Mobile or another U.S. GSM carrier isn't completely clear, since I haven't tried it, but my source at Apple seemed to indicate that if the frequencies match, then it will work. Incidentally, it's worth paying close attention to the rate structures on the 3G/4G iPads before you sign up. My analysis indicates that you get a better deal from Verizon Wireless, but then, this is being written before the new iPad actually arrives. The rate plans could change once that happens. It's also worth noting that I was told by a Verizon Wireless source that the company doesn't require U.S. users to use their data service, but that they think most users will want to. Considering Verizon's commanding lead in LTE deployment, Verizon is probably right about that.
This is important for a couple of reasons. The first is simply about money. Data roaming with a U.S. SIM card can be expensive. Those stories you hear about unwitting users running up bills of thousands of dollars for accidental data use while traveling aren't as rare as you might think. If you plan to use anything but WiFi while out of the United States, you can save a lot of money by picking up a SIM card for the country you're visiting.