The roaming data plans overseas are very expensive. The Wi-Fi should be no problem; but the cost per Gigabyte if you let your phone "roam" can be a multiple of what you'd ever expect.

A while back there was an article in the Wall Street Journal which sited several people that had many hundreds of dollars of charges that they were very dismayed and surprised to get.

You phone, left in it's own default setting streams data in many cases. Attachments to emails routinely are transmitted in some cases. As you surf sites, you stream pictures, video, etc.

I'm not an iPhone kind of guy, but you should see about turning-off the background stuff -- or just don't turn the unit on except when you're in Wi-Fi mode.