I've often wondered why this hadn't been done…
From I New Idea:
LG Display has developed a 14.1 inch LCD panel for notebook PCs that are illuminated by sunlight instead of the backlight unit (a backlight unit is an illumination source used in LCDs. It is usually made up of several fluorescent lamps, a light guide reflectors, and brightness enhancing films) when used outdoors. This is the first LCD panel to allow users to easily switch from backlight use (transmissive mode) to outdoor reflective mode with the touch of a button. The display's outdoor energy consumption falls to one fourth the level of indoor energy consumption, providing a significant increase in battery life.
In addition, this new product resolves visibility issues with a contrast ratio exceeding 9:1 when used outdoors in reflective mode. The display's contrast ratio ranges from just 2:1 to 3:1 for conventional notebook PCs when used outdoors, implying difficulty in viewing the screen. During CES 2009, LG Display will showcase its new 14.1 inch sunlight Illuminated LCD for notebook PCs, as well as its newest cutting-edge display technologies featuring improved motion picture response time (MPRT), eco-friendly displays and more at the Bellagio Hotel.