it's not about that at&t are compatible, it's about roaming. it means that at&t has roaming-partners (who provide access on their networks for your at&t-phone) in 190 countries.
if my phone uses the same gsm/3g/whatever standard as a network in a foreign country, doesnt mean that i can use this network with my phone. -> only if my provider/network has a roaming-agreement with this particular foreign network.
Exactly and AT&T has particularly diverse roaming agreements and particularly large usages on those networks so it has buying power worldwide. My AT&T service let me call UK really cheaply from my cell, and roaming at least within the USA was better than most networks.
I am presuming other countries are actually more fully deployed and sophisticated since we have so many regulatory and geographic issues here.
Rocketman