Good news!
Just because Apple ( SJ ) say something isn't possible, doesn't mean thats reality. If you've already paid for a data plan, you really shouldn't have to pay another data plan simply for another device ( thats along the lines of the record industry - pay for CD content, then pay for digital version etc etc ).
Of course, the 3G model will be more convenient, without the hassle of having another device with you, and shorten the battery life of the tethering server device.
I was thinking that Apple may have disabled Adhoc WIFI connectivity on the iPad, since thats what a lot of tethering software use - Adhoc instead of Infrastructure.