Switching off Cellular data in iOS4

This is more a confirmation than me having an actual problem. Anyway, I just updated my 3gs to iOS4 GMC and I'm wondering whether switching off cellular data over wifi will prevent my iphone from looking for a data network even if I take the phone off airplane mode. I'm about to spend a week in Japan touring with my family and we require a phone to use while there. However, I don't wanna pay $100+ due to my phone sending data coz I need keep my phone on during our vacation.

In short, the question is whether I will be able to use my iPhone in Japan without having to pay tons of $ in data fees after switching off all cellular data.