Yes, but you're still not following the topic. Nobody's disputing the applicability of an unlimited plan to 1st gen devices. What nobody can tell you with certainty - AT&T included - is what will happen with the next generation.
Right. The question of whether or not ATT will continue to offer unlimited plans on the next version of the iPad is purely a business decision that ATT will have to make. Some of the considerations for ATT--competition from Verizon, offerings from cellular-to-wifi devices, etc.--have been astutely identified by other posters, but it is ultimately ATT's call to make based solely on what they think is in the best interest of ATT. The decision isn't so binary either--they can offer a range of price points and data transfer limits, including, say, a 10 or 20 GB/month plan for the same price as the current unlimited, an offering that many iPad owners might regard as virtually unlimited, but would nonetheless let the camel's nose inside the tent.
It's interesting to speculate on what business factors may influence, if not determine, ATT's iPad pricing decisions, but there is no decision that is mandated by any consumer contract or by past practices. I do wonder, though, how much input into pricing Apple may have. The demand for iPads generally, and certainly for those with cellular radios, must be meaningfully influenced by the perceived cost of cellular data, so Apple would have an interest in keeping that cost down. Maybe when ATT has exclusivity Apple can negotiate stronger influence for itself? At some point, when there are many cellular providers who can provide data to iPads, Apple can rely on competition to control pricing. I wonder whether the negotiators of the ATT/Apple contract for iPads anticipated the mobile hotspot technology as a competitor to ATT, and what real competitive effect it is having now. I wonder if Verizon offering an unlimited mifi contract would influence ATT to maintain its unlimited cellular offering, or if the weaknesses of mifi (another device to tote, battery limitations, etc.) leads ATT to dismiss it as a serious competitor.
The whole transition for ATT from exclusive provider to just one provider among many will be interesting to watch, and the effect on unlimited plan holders not the least of reasons why.
