When users buy Apple's new iPad (the company has deliberately shied away from calling it the iPad 3), the device will be a world-class tablet in more than one way. Not only will it have world-class resolution on its much-hyped Retina Display, but it will be faster and have better app support than most other tablets out there. But perhaps the biggest improvement is the fact that the new iPad 3 will be usable in more than 200 countries, which basically means you can use whatever version you buy anywhere in the world.
As has been the case in the past, the 3G/4G version of the new iPad will have two flavors, one that works with the Verizon Wireless data network, and one that is designed for AT&T. Both will come with SIM cards for use on GSM-based systems and for Long-Term Evolution (LTE), which also uses SIM cards. The biggest difference is that the Verizon Wireless version will support Verizon's CDMA-based data network as well as LTE, and it will also support GSM-based data networks, including HSPA and UMTS.
The AT&T version won't support the Verizon CDMA legacy networks, but will support its existing GSM-based network, as well as its nascent LTE network.
If you look at the Websites for both carriers, you'll see that they both tout the ability to use the iPad globally, with Verizon claiming more than 200 countries and AT&T claiming slightly fewer. I assume this difference is because there are places in the world that also use the legacy CDMA standards, including some parts of India. Neither company was at the point of providing a definitive list, but that's not surprising. It is a little early after all, and the new iPad hasn't even shipped yet.
What's important is that just because you're traveling abroad, this doesn't mean you have to use the SIM card that both AT&T and Verizon Wireless provide with the iPad. While you can use it, and be billed on your carrier phone bill, you can also buy a local data SIM and use that. This means that if I were to want to use such an iPad in Germany, where I was recently attending CeBIT, I could have purchased a SIM from T-Mobile Germany, which is now deploying LTE, and used that.