If you're using Wi-Fi only, you will not be charged for data. To make sure your iPhone won't sneak by Wi-Fi and use the network instead, you should turn on Airplane Mode and then turn on Wi-Fi separately. AT&T, nor any carrier to my knowledge, has the ability to track your data usage when you're on Wi-Fi only and not on a cellular network. In Airplane mode with Wi-Fi on, your cellular radio is completely off and AT&T will not know what data you're using, much less charge you for it.
You should also check out AT&T's global roaming page specifically for the iPhone
here, if you haven't already.