AdCenter is available in many countries (like UK and Ireland for instance), so what is your real question?
We've used them in the UK and were reasonably impressed, the editorial process can though be a little slow.
Actually I recently registered an adcenter account and I live in a third world country. Didn't have any problems so far and ads are displaying correctly...
they are merging with yahoo in the next two years, maybe they'll add contextual ads from the yahoo network. As a foreign advertiser you can definitely join microsoft adcenter. But as a publisher only US residents are allowed.