You may be ok, but call T-Mobile and ask if you've charged data fees. Don't wait for your bill.
The reason I say this is I found the APN settings were already configured on my 9700. I don't know if that's the general case, or the settings were automatic because of my data plan. Previously, I and everyone I know had to manually enter the settings.
So, make sure you haven't been using carrier data throught the apn at a per kb rate and didn't know it.