These new cards make use of the SDXC format that has a theoretical limit of 2TB, rather than the current standard SDHC format, and so most current electronics may not be able to make use of the extra storage. That said, most SDXC cards are compatible with the older and more established SDHC, but you won;t be using them to their full advantage.
However you look at it, fitting 64GB onto something the size of a finger nail (15x11x1.0mm to be precise) is a pretty impressive feat in itself.
Lexar already produces a 128GB SD card, the microSD’s big brother.