Let me try again,Then it's not unlimited
Right now, data is unlimited for the iPad. As AT&T tracks its bandwidth usage, if they see that they're losing money on unlimited 3G for $30/month because people are "serving up wifi for every other device", then it would make sense that they change the terms to extend their usual 5 gig "soft cap" to the iPad deal. People may be grandfathered in, but since it's a month-to-month deal, AT&T is only on the hook for 1 month or less. In other words, if people abuse the plan they change the plan. Just like they did with their other data products that DO have a 5 gig cap.
As it stands, I'm sure they've already figured out your scenario as a possibility and accounted for it. AT&T has already stated that they think the 3G use on the iPad will be relatively low, so I suspect they feel they don't have that much to lose (or gain). It will be interesting once the 3G model is released, to see what kind of sales they generate. Both Apple and AT&T have stated that they're inclined to think they'll be lower than the wifi only version.
So, from a technical standpoint tell me how we go about getting the iPad to "serve up" 3G for all those other devices....