As everyone mentioned above... not for Pre-paid! I live all year long in Japan and my T-mobile is 24/7 Internet is slow, but it does the job and I can do everything I need from my mobile phone.

If you dont have the international plan activated, you will have to do this before you travel.

And the link below is the official Flyertalk T-mobile post... T Mobile Global data coverage

I hope this info is not too late, as you did not mention when was the trip!