I want to echo that the AT&T iPhone is hugely disappointing when traveling internationally due to the outrageous cost of AT&T's roaming plans. Data is prohibitively expensive. Think $1 per megabyte (not a typo, mega-byte). That's $200 for 200 MB or $2,000 for 2 GB. Voice calls are usually $1 per minute. You're pretty much stuck with a very pretty machine that sends $0.50 text messages.
Even if you get an unlocked phone, you will most likely only be able to get a voice+text SIM card when traveling. Especially in Europe, only full blown plans included any amount of data. And every damn country requires a new SIM. Absolutely painful and honestly not that valuable. You might just gain ever so slightly cheaper calls and texts to local phones, and absolutely nothing if you want to call back home.
Verizon Global phones, on the other hand, are amazing. AMAZING! You pay $80 per month for unlimited data. Yes, UNLIMITED international data. And Verizon will prorate your fees for only the days that you travel and also discount you the cost of your existing data plan during these days. Otherwise it is the same as AT&T, but this is a HUGE difference.
If you travel and want to use data then Verizon is the way to go. You just need a Global phone, and unfortunately the iPhone isn't one of them.