So I have read through every post on this topic to get everyone's view on the subject. now here is why I voted yes to a DLC outside the US.
Even though America might have closed themselves off from the rest of the world doesn't mean that other countries didn't loose all of their collective intellect. they would have had their own scientists and inventors. there is also several hints through out the game series about possible spy activity. which could mean that some of the American tech everyone is saying wouldn't exist outside of the US could have found its way into the hands of said scientists/ inventors. The other countries didn't fall to third world status just because America closed itself off.
possible locations more than likely wouldn't include island locations such as Hawaii, Cuba, Dominican Republic as the seas "boiled up" and would have totally decimated everything on said islands.
I understand wanting to know how "the other side is fairing" however an American showing up in China would probably be a really bad thing. As the American mindset in the game is geared towards fear and hate of Chinese. I would think that the Chinese feel the same towards Americans. I personally wouldn't want to finally make it all the way to China just to have the whole country after me.
Going off the basis that other countries moved along at the same tech pace as America and that there were spies going both directions, one possible bases for a DLC would be the BoS finding a bit of intel about an interesting piece of tech in another country that they send you and your companion to retrieve. they would drop you at this location via worrlybird (sp?) and arrange pick up once mission is complete.
( possible spin-off) Another form of transportation that has already been mentioned is the spacecraft. As of the end of FO3 it was still up there waiting for you to return for more fun.
To the argument that the basic theme of FO is America during the 50's (sort of), during that time in history there was a lot of travel to other countries, both for Americans and others, there was a lot of mixing of cultural influences. So some of the background items/scenes/music may change a bit, but overall the 50's where the 50's.
Oh, and one more thing. there is no way a project as big as the Vault system could have been completed without joint support from foreign entities, so those other countries could of had the tech to do similar things in there countries.
Sorry for the long post