
Originally Posted by
carsncars
It's the type of LCD panel they've used. Screens like that of the K800i, K790a, and W800i are much more reflective (you'll see they're grey when they're off) whereas screens like those on the W580i are much less reflective (black when off), so they're almost illegible without the backlight (but they are more vivid when the backlight is on).
Since the sleep mode clock would be useless with this type of LCD, Sony Ericsson figured they might as well disable the feature.