I recommend using a Tru SIM international SIM card for international travelers (it is inexpensive and convenient), but I also understand that many people just turn on their phone and roam with their US wireless carrier when traveling. Thus, it was great to see AT&T lowering their international roaming rates effective tomorrow.
You can roam internationally with all AT&T and T-Mobile phones, but have to make sure you have a global device on Sprint and Verizon before traveling. Here are the current and new data rates for AT&T where you can see new savings ranging from $20 to $40:
Current: 125 MB for $49.99 / New: 120 MB for $30/month
Current: 275 MB for $99 / New:300 MB for $60/month
Current: 800 MB for $199 / New: 800 MB for $120/month
The overage rate for all packages is $30 for each 120 MB. Make sure to check out the AT&T rate calculator before traveling so you can calculate your call rates and then compare to Tru SIM.
In comparison, Verizon has a global data rate where they charge $25/month for a 100 MB allowance. T-Mobile and Sprint are not friendly at all to the international traveler with data roaming rates on the order of $15 to $19 for just a single MB so don't even think of using their networks while traveling. It isn't that easy finding the Sprint rates, but I found another page where you can buy data packs for different countries for a cost of about $10/MB. Still outrageous compared to AT&T and Verizon though.
These are still quite expensive and if you roam via your wireless carrier you will have to pay very close attention to data controls or your bill will be huge. Again, I recommend you take a look at something like the Tru SIM or even a SIM card from the country you are traveling in. I previously detailed how you can use a Verizon iPhone 4S internationally so make sure to prepare in advance for your trip overseas.