I did remove it from /data/app and it's still showing up as 'installed' in the market... it seems to not have broken anything.
If anybody is interested, I just realized why mv doesn't work. /System and /data are on separate logical partitions (think two different hard drives). Mv, on a single partition, simply moves the pointer to that file, without moving any file data on the partition. Since the mv would try to move to a partition where the data does not actually exist, it fails. Cp, on the other hand, copies each byte of data to the new location, so the data will exist properly on the /system partition.
Makes total sense now.
If anybody is interested, I just realized why mv doesn't work. /System and /data are on separate logical partitions (think two different hard drives). Mv, on a single partition, simply moves the pointer to that file, without moving any file data on the partition. Since the mv would try to move to a partition where the data does not actually exist, it fails. Cp, on the other hand, copies each byte of data to the new location, so the data will exist properly on the /system partition.
Makes total sense now.