As chromatix has a couple of times mentioned e-ink displays, I had a look, and found this project by Jan Ostman:
e-ink works by rotating tiny bi-coloured spheres using electric fields, and is non-volatile so uses zero power in between updates. You can also get tri-colour (black, white, and red) displays.
And according to commentators over on the hpmuseum and on the eevblog forums, the DM42 calculator display is a non-volatile or micro-power LCD from Sharp, which again can hold a static display with (next to?) no power.
Quote:
Assuming that the LCD in use is a LS027B7DH01 from Sharp
Any other experience of zero-power displays?