A cavity shader makes it several times easier for modelers and sculptors to easily visualize bumps, valleys, and detail. This is where concave areas are displayed in a different color with a lower specularity.

example of zbrush's cavity shader:
http://www.shuma.fr/image/alienball/alien_ball_bronze1.jpg

Having used sculpt mode heavily for the last year, I think this would be incredibly helpful for artists.

There are three things that it needs to do:

1) update in realtime in the viewport while sculpting

2) work well with matcaps

3) be able to bake to a UV map