I think it might be relative to where you go, actually. I contacted T-Mobile about the Philippines, and I was explicitly told that, yes, I would pay for data. The first megabyte is included with the plan, but then it was rated at $15 per subsequent megabyte.

Strange, however, that there seems to be a lack of specificity in the T-Mobile hierarchy. With many people receiving conflicting answers, and such, it's rather annoying. I suppose I may not conclusively know whether or not T-Mobile will charge me for data until I actually use it.