Depends on the country. In Japan, at least, I've bumped into merchants with country-locks (oddly, 7-Eleven is the biggest offender in this regard), and in every case, my US-issued AmEx slips right through, as did my China-issued JCB (while I had it, at least). I'm told that it's due to the way they process transactions that they can't be locked out. I wonder if this is the case elsewhere (for AmEx, at least- JCB is pretty much non-existent in Europe).