I was going to ask what the point of this was, because I can just do it already on my iPhone. But from the comments I guess they charge you extra for it in the US?
That sucks.
T-Mobile: Unlimited txt/voice, 5GB data == $89.99 USD (2GB is 10 less, there is no cost overage after the cap, but the speed drops to less than 2G)
Sprint: Unlimited txt/voice/data == $110
ATT: Unlimited txt/voice, 4GB == $115, 2GB == $95