[spuggy]

GSM is a standard, and even though the US didn't implement quite the same as the rest of the world, any quad-band GSM phone works fine - for voice - anywhere there is GSM service. Which rules out parts of Asia and Africa where the only services available are from the older equipment we decomm'd, but developed countries should be fine.

You may need to enable international roaming with your carrier for your phone to work abroad. This is probably no-charge - because the carrier makes plenty of money (mine charges $1 per minute) on the calls you make/receive. If you can bear this price, it's the most convenient - because your "real" phone # Just Works. Be aware you need to add/dial "+1" in front of regular US phone #'s for them to work (and the phone/operator is smart enough to use that to dial the international dialing code). All the phone #'s in my phone all have the appropriate country codes...

Beware data charges on international roaming rates. These can run into $1,000's of $$$ (one guy famously ran up something like $50,000 in a week on a cruise ship just off the coast). Make sure you know what your charges are/how to disable data when you don't need it.

Depending on the frequencies your phone is using for data (which depends on which US carrier it was originally supplied by), you almost certainly won't get 3G speeds anyway (different radio frequencies), and will be limited to GPRS/EDGE.

If your phone were carrier unlocked, you would have the option to remove the US SIM and buy a local pay-as-you-go SIM - but you're probably not that interested in making local calls; costs for international calls on that SIM would vary by plan; a pay-as-you-go plan wouldn't give the greatest rates - but probably still cheaper than $1 a minute.

Some carriers will provide the PUK to carrier-unlock freely (T-Mobile will give it out once your account has been 90 days in good standing), or it can be achieved by other methods. I'm told that AT&T don't like to give it out, ever. On an iPhone, I think you need to jailbreak to remove the carrier locking in any case.

VoIP application like Vonage or Skype is probably best for calling - but you probably only want to use these/may only work properly (due to data rates) on WiFi.