We are in the third generation of the iPad and they have not once increased the storage space. This used to be a usual upgrade with the classic iPod.
They've never upgraded it because sales of the 64GB version are the lowest of the various models. You can't make a case for more space when people aren't using the max. I'd love a 128GB version, but it's not going to happen until the take rate improves. For now, I get by with my AirStash and 3 32GB flash cards.