From AT&T's website.
"For just an additional $24.99 per month, iPhone customers may add a Data Global Plan to their existing domestic data plan and receive 20MB of data usage in 29 countries, including Canada, China, Mexico and many additional countries in Europe/Asia. Overage rate is $.005/KB. Outside the 29 discounted countries, the data usage rate is $.0195/KB"
I don't know how many months you have to buy minimum.