Unlocking your GSM phone

Popular Science has this article on unlocking your GSM phone so that you can really be number-portable. This works great in theory, but after I unlocked my Sony T616, I realized that I couldn't really use it in India (or anywhere in Asia) since AT&T uses the 900MHz frequency in the US, while the rest of the world uses 850/1800MHz. So while the T610 (850/1800/1900) will work fine anywhere in the world (rest of the world really), the T610 (900/1800/1900) won't. Well, so much for tri-band.