Get free international roaming turned on. You have to ask AT&T first.
It is NOT $20/Mb if you plan ahead. AT&T sells international data packages for 120 Mb for $30. Or 300Mb for $60,etc. They are for the month. Actually you can bump up your plan while traveling with no extra charges. Start low and see how you do.
If you do get it unlocked, and pop in a foreign sim, there will be settings to change from the foreign carrier. And if you hit multiple counties, the data charged may add up to be more than AT&T rates!!!
AT&T has a good website for traveling with an iPhone. Also there is an International AT&T phone number for service.
Also get AT&T World Travelerfor cheaper voice rates. Only $5.99 a month.
Believe me, I found traveling in 2 european counties for 2 weeks with AT&T cheaper than using local sims. The terms of the data plans were the killer. If it was only one country then it made sense.
Each case is different. Check before you go. if you don't expect using much voice, a local sim may not be worth it
Don't forget that stateside people can call your local number for free, with AT&T. However you need to call or txt everyone with a +1(area code-number). Even to your travel partner