Hi rigormortis,
Some of our partner stores, such as Best Buy, Radio Shack, Wal-Mart, etc sell SIM cards seperately from traditional plans. They choose what they want to charge for their product as they are not part of AT&T (This is why sometimes you will see one of these locations have deeply discounted devices opposed to the AT&T Stores)
Most AT&T Company Retail stores are now selling the SIM kits for tablets for $15, but it also comes in the box with a $15 refill card (effectively free).
These kits have adapters in them for all sizes of SIM and will work with any iPad device. I believe that person you spoke to or the package you read was mis-informed/mis-typed. These sims are usually marked with a green "thumbprint" meaning they are set to activate once put into a tablet device.
Hope thie helps!