There are 22 addressing bits available on the SDHC cards, only 16 of them are in use by 32GB cards. Any card that actually sets any of the above-16 addressing bits will work out of the box in Linux, i.e. up to 2048 GB. The kernel checks all the 22 bits, not just the 16 that are "officially" allowed (and which defines the 32GB limit).

Presumably the SDXC cards use exactly the same addressing mechanism, but they may use/support a higher frequency (for better throughput) and we already know that they standardize on a different version of the FAT filesystem.

In short, nothing[1] stops anyone from making bigger SDHC cards, and they won't get into the kind of hairy compatibility problems that the non-standard non-SDHC 4GB SD cards had.

Footnote: Nothing technical. Political etc. is another possible issue, see next poster about possible Sandisk squeeze control.