iPad users don't have access to the "overage" rates.
Not according to endgadget....
DataPlus / DataPro
The existing $30 fair-use "unlimited" smartphone data plan is being replaced by two new options: $15 per month for 200MB and $25 for 2GB (called "DataPlus" and "DataPro," respectively). Customers currently on the $30 plan are welcome to stay on it, but they can switch at any time without extending their contract.
AT&T's new overage system is arguably the game changer: on the $15 plan, you'll pay $15 for each additional 200MB, but on the $25 plan, you'll pay $10 for each additional GB. It's simple and straightforward -- but most importantly, it won't bankrupt you if you go over by a gig or three in a month. This compares to $50 per gigabyte of overage on AT&T's 5GB DataConnect plan for laptops.
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iPad users are also affected by the change. The $30 iPad data plan -- lauded for being labeled by AT&T as truly unlimited -- goes away to be replaced by the same $25 / 2GB plan that smartphone users will see, though current subscribers to the $30 plan can continue unaffected
http://www.engadget.com/2010/06/02/...-changes-to-data-plans-iphone-tethering-comi/
EDIT: Sorry - didn't read the TUAW story before replying.
You are right.
That strikes me as a rather bizarre way to do it.