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.