The biggest limitation is the RAM. With only 256MB, the iPad has little room for a bigger OS. Many of the new features in iOS 6 will require more background tasks and buffers. As some have noticed iOS 5 is already taking the iPad 1 to the edge ,RAM wise. Yes, the 3GS has the same amount of RAM. However applications for iPads are significantly bigger than iPhone apps, especially since the introduction of the new iPad with it's retina display. And as I've said before, from what we are seeing the 3GS is pretty close to getting iOS 6 in name only. None of the new-cool features are going to work on it. So the alternate theory (in leu of "Apple is just greedy") is that Apple is not going to put iOS 6 on the iPad because it would result in performance so poor it would piss customers off even more than not getting the upgrade at all. Reality is probably somewhere in between, as it usually is.