You're not going to get LTE overseas with an AT&T phone. But LTE is really in its infancy in much of the world anyway, so you just allow yourself to be satisfied with your data coming in at slower speeds and enjoy being able to use your iPhone wherever you are visiting on a local network.
In other words, I will get some sort of internet access, 3G or whatever the local carrier offers, just not the LTE like in US, correct?