Apple made it very clear that Flash would not be on iOS devices. Also, consumers are spending over $500 on iPads. For most people, that's not just chump change. It's a substantial amount of money it's consumers' responsibility to do their due diligence before buying the iPad. If only a few thousand people had bought the iPad knowing it doesn't have Flash, then it could be said that that is too small of a sample size. One can argue that those few thousand people are a special case and one can't extrapolate what the general consensus is. But 25 million buyers is a substantial number by any standards. If 25 million people still bought the iPad knowing that it doesn't have Flash, what do you say to that?
Right now, the iPad and iPhone are much superior than everything out there, so people will buy it despite it not having flash. I want to be able to use flash, but I still bought the iPad and iPhone because everything else it does, it does better than everything out there right now.
But, if flash doesn't completely die off soon, things could get interesting as the competition catches up. Webos is already a superior ui, it just doesn't have the hardware or app support, and windows will be a strong alternative as their ui is on the same level as apples, it just whether you prefer a list view or grid view on your phone/tablet that differentiates them. And when windows 8 comes out and ties together windows phone/tablet/and desktop thing could get really interesting fast.
Flash alone won't get anybody to switch platforms, but if the competing platform preforms just as well AND has flash support while apple doesn't, than you can see people begin to choose the other platforms over apple's.