correct me if I'm wrong... but isn't the screen on the N900 a transflective screen? That means it can be seen in direct sunlight. Which also means that if you completely shut off the backlight, you should still be able to see the screen with enough ambient reflected light. I think when we lock it, the screen should be completely off (backlighting and display) but if you tap the screen maybe it wakes up to show you the clock with display on but no backlighting. Or maybe just always have the display on but backlighting turned off when locked.