The first-generation iPhone generated unexpected, exorbitant data transfer bills for some overseas travelers -- a result of automatically downloaded, non-user-triggered data and expensive rates (up to $20/MB in some countries). The issue was later resolved with the addition of a switchable "data roaming" option.
Now users are encountering a different international data transfer billing issue that can, likewise, result in huge, unexpected bills.
iPhone Atlas reader Doug Toombs traveled to Bermuda with his iPhone's data roaming option turned off. He returned to discover a $113 data transfer bill. Here's what happened: GSM networks work with of a series of Mobile Country Codes (MCC) and Mobile Network Codes (MNC). These codes are each three digits long and uniquely identify the country and carrier. Used together they can pinpoint the country you are in and the carrier you are using in that country. The country code for the United States is 310. This code is supposed to be endemic only to the United States. However, Digicel Bermuda uses the 310 038 combination. This caused Doug's iPhone to register on an ostensibly US-based network and ignore the data roaming setting. Despite the matching country code, Doug was charged for international data transfer.
AT&T forgave the charges, but other users have reported a similar issue, receiving bills upward of $1,000.
As a precaution, don't trust your iPhone if it shows a carrier logo matching that of your home country while traveling abroad. If such a logo is displayed, contact your carrier and ask for coverage details regarding the area.
AT&T has not yet responded to inquiries regarding this issue.