AT&T doesn't support LTE when roaming except in Canada. PITA.
International GSM versions can't get AT&T LTE in the US. PITA.
The only plus side to anything is that if you switch to Verizon you can use some international LTE and get an international unlock after 60 days. However, in my case, switching to Verizon just for international LTE support is frivilous since I'd be paying more for less data on a Data Share plan versus keeping my "unlimited" plan + other grandfathered options.
TL;DR — everyone is getting fucked by the iPhone 5 if you plan on using it in a different country at some point, or like simultaneous calling and data.