So what's the best way of taking an iPhone abroad? If you're really scared about running up data bills – and you should be – then one way of ensuring that can't happen is to phone up AT&T just before you leave, on 800-335-4685, and ask them to disable your data plan. Then phone them again on your return, and get them to turn it back on. You can still use the phone to surf the web and check your emails when you're in a wifi zone, but you won't get a massive bill for doing the same thing over the cellular network.
The other thing you can do is switch your phone to airplane mode most of the time. That turns off everything: both voice and wifi. When you're in a wifi zone, or when you want to make a phone call, come out of airplane mode and do whatever you need to do, then turn airplane mode back on again.
This solution has the advantage or disadvantage, depending on how you look at it, of barring incoming phone calls. You might miss something important, but you won't be woken up in the middle of the night by a telemarketer and have to pay over a buck a minute for the privilege. And every time you turn your phone on, to make a call or to surf on a wifi network, the iPhone will check to see if you have any voicemails from people who've tried to reach you. You can then check the voicemails you want to check, miss the voicemails you want to miss, and only pay for airtime once. If you keep your phone on at all times, in contrast, then you need to pay twice for every voicemail you listen to: once when it's left, and once when it's retrieved.
Your best friend, then, when you're travelling, is the Settings page. At the top of the page are the two most important settings: Airplane mode, which we've already covered, and Wi-Fi. If you want to use Wi-Fi, it's a good idea to select and join a network first, before you start checking email or web pages. That way there's no gap during which AT&T can start charging you their exorbitant data rates.

Taking your iPhone overseas can be a costly proposition, and you may not even know that you're racking up a huge data transfer cost until you get your AT&T bill. The problem is a combination of the following two facts:
1. The iPhone is capable of using international data roaming, which is billed at $5/MB in select countries ("DataConnect Global" countries), and nearly $20/MB in other countries (you must activate a per-use international roaming data plan)
2. The iPhone will automatically download data over available data networks without notifying the user for applications like Stocks, Weather, etc and other purposes which we can't yet identify. Even if a user deletes all companies from the Stocks application, and cities from Weather application, data transfer may still take place. (You can check data transfer stats in the in the Usage section of the Settings application.) In other words, even if you have turned off off all options you think could result in data access and don't directly access any data functions, you could still find transfers of several hundred KB on your bill.
So some users who have taken their iPhones overseas and used only voice functionality, or thought they were using only WiFi-based network access are receiving significant bills for international data roaming. There a few ways to obviate these charges:
Take the SIM card out of your iPhone. This will allow you to use the device to access WiFi services, but will disallow any EDGE/GPRS based data access, eliminating the potential for unwanted charges. You won't be able to make calls, however.
Call AT&T and ask them to temporarily disable EDGE access capabilities on your account while you are traveling. Use this number: 916-843-3685. Unfortunately, we haven't found a way to manually disable EDGE data access on the iPhone.