Squawk,
T-Mobile have the best international rates of the US GSM carriers both in terms of making international calls from your phone as well as making calls from your phone whilst abroad.
To roam internationally, you will need:
1. A tri-band phone (Ericsson T68, Motorola v60 or v66, Siemens S40, Nokia 8890 [actually dual-band but it would do]) These phones are commonly available in the US market.
2. International roaming needs to be enabled on your account.
Something that might be worth exploring is to get a pre-paid SIM card when you are travelling. For example, I bought a Dutch pre-paid SIM card when I was in the Netherlands recently. This cost me $20 and gave me $5 worth of credit (which could be topped up). The advantage is that calls cost much less since you are no longer roaming. The disadvantage is that your phone has a new (local) number for the duration that the SIM card is in the phone. Note that in order to use any SIM card other than the one provided with your phone, the phone must be "unlocked". T-Mobile and Cingular will do this for you. AT&T have been much more restrictive.
The biggest downside with GSM in the US is coverage. You're fine in metropolitan areas but there's much less coverage in the outside cities and highways.
If you need more info, send me a private message.