The simple one:
Buy a device from AT&T/T-Mobile for domestic use and I believe ALL the new ones have the foreign roaming 2100MHz band (GSM 3G IS CDMA!) so you just need to get it unlocked if the model works.

Or BUY the SAME model from AMAZON UNLOCKED and it will work domestically & internationally, just swap the SIM cards, but for data you need to manually configure the routing APN (The MiFI has a pre-populated list, if it doesn't have the carrier profile you have to manually edit it/add it on the embedded management web-server and the APN can be received from the operator's help website).

If you have a GSM 2G+3G device that has the 800/850/1800/1900MHz bands of 2G (EDGE speeds only) and 850/1900/2100MHz of 3G (UMTS/HSPA+ speeds) it will work in EVERY COUNTRY in the world /w coverage.

Just buy this Novatel: http://www.amazon.com/Novatel-Wirele...3283123&sr=8-2

Same as AT&T version, just unlocked from OEM and you can use whatever carrier you want worldwide (including AT&T /w their SIM card!).