df honors the BLOCKSIZE environment variable so you can already do things like:
$ BLOCKSIZE=1m df
Filesystem 1M-blocks Used Avail Capacity Mounted on
/dev/sd1a 1008 175 782 18% /
/dev/sd1m 124124 29107 88810 25% /home
/dev/sd1d 4035 0 3833 0% /tmp
/dev/sd1g 2413 2110 181 92% /usr
...
- todd
