Not the case with the original iPad plans that were sold for the first three (?) months after the the iPad 1 was introduced.
I had unlimited long before iPhone was introduced. Back then, phones were simply incapable of consuming anywhere near 4GB and 3G was still pretty new thing. Indeed, there was never a limit back then. iDevices in particular have a special waiver that AT&T will give it a priority over other devices when it needs to. But the cap was introduced later when 4G came along. For AT&T, it was 5GB, not 3GB.