SanDisk is launching their 1TB Micro SD Cards in April for $450 USD. That'll be the highest commercially available Micro SD card capacity at the time of release. I don't think one can easily justify spending that exorbitant amount of cash on storage, though.
Edit: Micron is launching one as well, not sure if that's coming sooner or later than SanDisk's, or what the difference in price is.