The Google Nexus One and Samsung SGH-i8000 Omnia II smart phones have Active-matrix organic light-emitting diode (AMOLED) screens. Web pages and apps using white text on a black background should save power on these devices.
The Blackle website, launched in 2007, claimed to reduce power consumption by reversing the usual Google search screen, providing a black background. The amount of power saved was not large, as most LCD screens have one back light which is on at the same brightness (and using the same power) regardless if most of the screen is black or white.
AMOLED screens have thousands of individual LED lights. For black, all lights are off. So using a black background should save power. This is significant for a battery powered device such as a smartphone.
The black background could be achieved for web pages and for applications written for the phones.
Currently AMOLED displays are prohibitively expensive for large computer and TV displays. But some larger LCD screens have multiple LED back lights which will save some power with a black background. For this to work there have to be large areas of black, as even one non-black pixel requires the back light for that area of the screen to be turned on.