I recently went to Zurich for a week-long business trip (training developers on Jive Clearspace). I knew I'd need to use my iPhone for a variety of reasons, such as email, web access, chat, and google maps.
So like any responsible employee I went into the AT&T office and spoke directly with a rep about what international data plans I'd need to purchase, and how much it would cost me.
I bought a 100 MB data package for $129.99, and paid a $5 international plan that brought phone calls down to $.99 a minute (otherwise they'd cost me $1.29 a minute). I reset my iPhone usage before boarding the plane, and watched it like a hawk the whole time I was there.
Here's what it looked like as I stepped off the plane at the Chicago airport for my connecting flight, on October 25th:
Figured I couldn't have cut it any closer, but was only about 5 megs over the limit, give or take a few megs that might not have been reported by then (although it was a 10 hour flight, so it should have been pretty accurate by then).
That next day, AT&T calls me to let me know I had a bill of $2350 from my data usage! I calmly explained that I had purchased the data plan and after a little digging and keyboard clicking on their end they were able to find that I had indeed bought the plan and only owed for the 14 MB of overages and owed about $54 extra for those kilobytes.
I wasn't sure where they came up with 14 megs, but the $54 didn't sound to outrageous and so I let it go.
Fast forward a week to November 1st. I go online to check my bill and find that they've tacked on $460 of data roaming charges. They're calling them mobile-to-web "calls" and told me I must have been in an area where roaming charges applied.
I went back and forth with them over it, explaining that I was never told to look out for additional roaming charges, and in fact was told that the international data plan would cover me for data usage through my iPhone.
AT&T finally agreed to split the bill with me, and apologized for the sales rep who didn't explain the plan well enough to me, and also pointed me to their online documentation - where they claimed it spelled all this out in clear detail.
So beware if you're traveling internationally with your iPhone!! Yes, the features are great and the google maps locator in particular saved my bacon several times. But you WILL SPEND a lot of money if you use it while traveling abroad, and I'd think twice about using it unless you absolutely have to.
Hopefully you'll learn from my mistake after experiencing this first hand, and please spread the word if you know someone who will be traveling abroad with their iPhone.