It's not really the item that you need to worry about, it is the carrier.  Working for the government, we have people that travel the world and they need exactly what you are asking for.  What will work in the US may not work in Europe or Asia.  So, your best bet is to contact the major carriers, Sprint, Verizon, AT&T, check out their coverage plan and consider how often you may travel outside your country.  From there, you can them make the decision on the type of hardware in which you can use.  Unfortunately, sometimes it is not easy telling a person that their Kindle will not work in Europe when they have all of their apps installed on it, especially Angry Birds...

It's not really the item that you need to worry about, it is the carrier.  Working for the government, we have people that travel the world and they need exactly what you are asking for.  What will work in the US may not work in Europe or Asia.  So, your best bet is to contact the major carriers, Sprint, Verizon, AT&T, check out their coverage plan and consider how often you may travel outside your country.  From there, you can them make the decision on the type of hardware in which you can use.  Unfortunately, sometimes it is not easy telling a person that their Kindle will not work in Europe when they have all of their apps installed on it, especially Angry Birds...