No more responses necessary. See below.

  slr> nor have i, my RH7.1, 7.2 and 7.3 boxes all report:
  slr> df -b MB
  slr> df: invalid option -- b

That was an upper case -B.

Someone sent me a copy of a df man page, said to be
from Mandrake 8.2, that does indeed document -B MB as
being valid. The relevant part of the man page says:

       -B, --block-size=SIZE use SIZE-byte blocks

       ...

       SIZE may be (or may be an integer optionally followed  by)
       one  of	following:  kB	1000,  K  1024,	 MB  1,000,000, M
       1,048,576, and so on for G, T, P, E, Z, Y.

Interesting. And certainly unportable.

-- 
Lynn David Newton
Phoenix, AZ