Apple is just going the route of making a toaster and hoping people will be upsold from the base model. Which is standard Apple procedure. The only thing going for the iPad is only capacity though. Are there any differences between the 16 GB and 64 GB models short of how much memory is soldered onto the board and the density of it?
Let me counter with this question: Is there any difference between a netbook with a 40GB drive and one with an 80GB drive? For some people, that's a huge difference, for others, it's not. If you ask me, 40GB drive is barely enough to allow for the OS and maybe one or two apps. 80GB at least lets me load a couple of movies as well.
The 64GB is double that of the current highest capacity iPhone/iPod Touch. That seems a lot when you consider that the average app is only a few Megabytes compared to the 100+ MB most Windows applications now run. To some extent, that 64GB feels more like 640GB by comparison.