What makes you think that it ever will?
For reading books, I think it's pretty irrevelent, to be honest. Text is just as clear on my 4 grey-scale Sony Reader as on my 16 grey-scale iLiad.
It doesn't even make that much different for displaying pictures, to tell you the truth; the dithering algorithm used by the Reader pretty much makes up for the extra "real" grey scales of the iLiad.