As mentioned previously, the AT&T iPhone will work in just about any country because it is quad band GSM and triband 3G. Well, to be precise, the AT&T iPhone 3G, 3GS, and 4 will work in Japan. The original iPhone will not.

The Verizon iPhone uses Verizon's network technology, which is far less common in the world. It will work in the US, Canada, parts of central/south America, Israel, and parts of China (and a handful of other countries).