Ridgebacks --
My SO and I have been AT&T/Cingular wireless customers for years. We travel out of country; destinations are generally Mexico, Canada and frequently China. We have experience with AT&T international roaming and Edge data international roaming plans. (We have an AT&T wireless card with data plan for our laptops for emergencies.)
SO has a Blackberry, which he relied on completely during a recent trip in China, when he spilled a Coke on his laptop inflight. The BB bundled data plan is more expensive than the iPhone plan, but it incorporates receipt of messages almost anywhere in the world. However, SO seldom uses his BB as a phone in China/Hong Kong, because AT&T roaming charges (phone) are high. He has a dedicated cell phone with a Hong Kong #.
As well, the wireless card is used out of country only in emergencies. Edge roaming rates do apply and are incredibly high, even in Mexico.
I doubt that AT&T will deviate very much from their historic billing policies for the iPhone. I have seen on some message boards that AT&T is offering an international data plan for use with the iPhone, but that one must purchase it for at least a year. There is no "purchase as needed monthly" option. I don't know what the rate is for that plan.
My suggestion, if you don't want to use airplane mode, is to turn your email accounts "off" under settings. That way, phone is functional, but the network won't download email. That's what I'll probably do next time I'm ooc.