iPhone data plans in Europe actually do have limits
It varies from carrier to carrier from O2 to T-Mobile to Orange it all varies, but they all have some rather strict limits. T-Mobile has three plans sized like your shirts, M, L, or XL with data limits of 200MB, 1GB, and 5GB respectively.
After you go over those limits your still have unlimited data, just your access speeds get throttled to 64Kbps instead of the normal speed of 220Kbps. That ad was taken down shortly after being posted, but if I had to guess it will still stay that way, just the fine print will be hidden somewhere else.
