Edit: I know it's impolite to complain about downvotes, but I'm genuinely curious. This comment is currently at 0 points, even though I just stated something completely factual without any loaded language whatsoever. What's the deal?
My tablet barely uses any data (with syncing enabled) in the background on cellular data. This despite having background syncing turned on and quite a few apps that use it. I take it with me most places and do casual browsing on the cellular connection (public transit and airports mostly), and the most I've used was 150MB/month. It just waits to do anything bandwidth intensive until it's back on wifi.
I'm really surprised Apple hasn't done this yet. I guess that's the reason why carriers complain iDevices use so much data.
So does iOS.
I didn't mean my comment to be a knock against Apple, as it was clearly my fault (first time with a severely limited data cap), but just to illustrate how easy it is to accidentally use up the small amount of free data T-Mobile gives you.
Obviously, the best solution is to magically eliminate the limitations of radio spectrum and give everybody unlimited data.... Until then, I think stories like yours will continue to be common.