I've been keen in unlocking every one of my GSM phones because of one simple reason: ability to use it internationally. Before leaving the country with my unlocked N95-3, I phoned AT&T to be certain on my usability and cost:
1. For starters, I was told that being on a plan for small business, my account is currently activated for international calling (somehow I got enrolled in the small biz program and the response rate and customer service has been superb, I think its because of that small biz connection).
2. For the voice calling cost, you can visit here or call AT&T to verify. As for me, the cost to call in Taiwan is going to be $1.99/min and when I travel to Indonesia next week, it will cost me $3.99/min. There are international calling plans available, but I am not planning on using the phone unless its necessary, so I am not looking to paying for such a plan.
3. For data plan and SMS Text, they will usually charge you per use. I am not planning on using any data plan while traveling, so I did not check on the cost. As for SMS, it goes to $0.50 per message.
As a general caution, be sure to turn of any data sync ability, to prevent being charged for data used unintentionally (ie. push email, or any other third party app running). As for voice calling cost, I would advise turning off your phone unless you need to use it. Leaving it on can accrue unintended cost as well, ie. your friends from back home could try to call you not knowing you are not in the states.