. Apple was smart enough to put in an override preferences for people who travel. You turn it on and the phone stops you from making calls or, drops them when you move into a roaming area that isn't supported, but can still work with the phone to avoid racking up charges.
And you should use that setting.
I was in Canada in December, and I was connected to the hotel WiFi. I was demonstrating how to download a 675MB movie to the iPhone for the client. When I put the phone down, the WiFi dropped, the phone switched to 3G and merrily downloaded the 672MB it didn't get via Wifi.
My overage was - for a few moments - thousands of dollars, but the folks at AT&T reduced that to "only" $200 for the 800MB Canadian data plan.
Note: Canada has the most insanely high data rates for phones anywhere