The maximum size allows by the SDHC specification is 32GB, but as some said, they're not out yet.
At least one reason these might not be out yet is chip size reductions.. fitting twice the flash in the same space. That could mean more power, but it could also mean less power, since a smaller chip geometry might be less of a power draw. Flash power is pretty small, anyway.
More interesting, there's nothing different electrically between SDHC and the followup spec, SDXC. There are certainly driver differences, and to be in full SDXC specs, you must run exFAT, not FAT32, as the file system. So it's not out of the question that we might see exFAT and SDXC drivers as an add-on or upgrade, to allow 64GB and beyond. Of course, at the rate they're doing with the micoSDHC 32GB release, it may well be the DROID2 getting that larger flash card.