In other words, Apple has removed the biggest pain point from the mobile / cellular experience for the customer: dealing with the operator.
Apple has also thus removed what is seemingly the worst pain point for the operator: having to deal with the customer. What, too harsh? I would think that if we weren't a problem to the operators, they wouldn't treat us like they do.
On a more serious note, I know that many Pay As You Go (PAYG) carriers show you your balance after every transaction, including browsing (using a technology called SSID, which is best described as 'SMS Lite'). This means that you do see how your data usage makes your allowance go down. I know of no subscription or monthly plan that does something similar and actually warns you when you are reaching your limit or cap on data usage; the usual thing for operators to do is to silently switch the consumer to a more expensive per-byte plan. This warning on the iPad may be the first real-time instant feedback a mobile device gives you that you are running out of data on your monthly plan.
Since operators really want to get rid of unlimited data plans, and this is a consumer-friendlier way of managing data allocation than silently charging the subscriber a mint for data overages, expect this to trickle down to all smartphones. All you need to do as an operator is have a really simple data plan with easily allotted blocks, and software on the phone that knows about them and can shop for more.