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.

O2 on the other hand has a fair use policy built in that basically says if you go over 200MB they can hit you with some extra fees or charges, but at least O2 had the decency to partner up with Cloud for a huge amount of WiFi coverage so you hopefully won’t be using EDGE much anyways. There isn’t any news on the Orange front, but apparently they are the priciest of the bunch in general and seem to have a tendency to just start charging by the Kb once the limit is exceeded.

So, regardless of who your carrier is, if you want to keep your bill down, I would do my best to keep data usage when not on a WiFi network down to a minimum. Or you could always start your own network and give yourself unlimited data usage, that would work too.

“Unlimited” iPhone data plans on T-Mobile, O2 and Orange not so unlimited [via engadget]