I have BIS and have recently upgraded to the 8700g. I previously had the 7100t, but for this discussion, I doubt that there is a difference.

I was overseas for 3 1/2 weeks between the end of December '05 and the beginning of January '06. I travelled from Singapore to Sydney on a Cruise Ship with several stops in between. My limited time on shore certainly kept my data rates low, but I was most certainly charged for data at the going rate for each country. I believe that the rate was $15/mb as you have stated. I signed up for the $20/month unlimited International E-mail service as well because I wanted access to send/receive E-mail while I was gone without the worry of how much data would be contained in each message. I basically signed up for 1 month and removed that service upon my return. So, yes, T-mobile charges for International data. Perhaps there is an even better plan that provides unlimited data access as well, but I am not aware of one.

Keep in mind, even though there is a per-MB data rate on chat, I suspect that you will find it to be much cheaper than using the phone or accessing E-mail for messages.