Only trouble is, most phones sold in the US come "locked" to the carrier which sold them to you. Case in point: the phone I bought two years ago, and the phone I bought two weeks ago, are both locked (by AT&T Wireless and Cingular, respectively). As locked phones these handsets will respond only to the SIM cards the companies provide. You can unlock the phone, and it's not all that hard (just key in a secret number based on the handset's unique serial number), but doing so usually will void your warranty. Plus, the US carriers don't seem eager to encourage this effort.
But when time is on your side great things are easy. I'm not traveling soon so I could easily afford waiting the week for an unlock code. I called Cingular, read them the IMEA number from the back of my old handset, and they said they'd e-mail me in about a week with an unlock code. Goodness only knows why it took a week to prepare this code, it's not like it's a magic spell that needed to be researched. But no big deal. Last week I got the unlock code and today I followed the directions to unlock the phone. Carefully, too; the directions said that, if I tried but failed to unlock the phone 5 times in a row, the phone would be permanently locked to the carrier. Great, permanently locked to a defunct wireless provider (AT&T Wireless). No worries, though; I got it right the first time. My old handset now happily accepts my new SIM card; I now effectively have two different cell phones (though only one SIM card). Handily, these phones also share accessories (they're both LG handsets). And, most importantly, I can use my old handset with foreign SIM cards. When I travel abroad, I'll take that handset, buy a pre-paid card from a local vendor, and enjoy having a nice handset that I know how to work.