ATT offers an international plan for data but it is expensive and isn't unlimited.
I would look for a prepaid microsim in the country you are traveling to.
From what I understand, you pay for 3G service via iTunes or some other app ahead of time and you can stop it at any time. If you have it enabled and travel overseas, how is your data rate changed?
I'm thinking with this business model the price is fixed wherever you travel.
I've looked into this a little already since I spend about half my year (or more) in Korea and Japan. It's data rate changes because you are actually using a different carrier overseas and AT&T's international ROAMING charges (which are clearly listed on it's site, and are extremely expensive) kick in.
The way to go is to try and find a carrier in your host country that is compatible with the micro-sim, that you can sign up with for the duration of your stay at a normal price.