> Dithering is used by imaging software to make a 256-color image look 
> like it has more color depth than it actually has.  In some images,
> the  256-color image can look exactly the same as a 24-bit original.
Not exactly. Dithering trades spatial resolution for colour resolution.
This is why it's popular for printing (where spatial resolution is much
more than it needs to be) but less so for displays (where spatial
resolution costs).

> So, my question is this: has anyone done or seen any research on 
> dithering a 48-bit image to 24-bit?
Who uses 48-bit images? I was under the impression the next stage up
from 24-bit colour was 24bits per channel as used for films.

> The ultimate effect is that a 24-bit image could look nearly as good
> as  a 48-bit one, and most importantly, the improvement would be
> visible on  commodity hardware.
I think 24bit images have more colour resolution than you or I do, and
that commodity hardware doesn't have enough spatial resolution to spare.
If you want more contrast in your pictures, you'd be better off IMO
starting from YUV images and dithering the Y channel to get more
luminance resolution.

Daniel

-- 
Now  as he walked by the sea  of Galilee,  he saw  Simon and Andrew  his
brother casting a spam into the net:  for they were phishers.  And Jesus
said unto them, Come ye after me, and I will make you to become phishers
of men.  And  straightway  they forsook  their  nets,  and followed him.

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