I second that and would like to add that turning down the brightness can make reading a lot more comfortable on a pc too.

My own findings after intensive testing with a thinkpad notebook are that a completely white LCD consumes measurably less power than a completely black screen.
Measurable, but far from being significant. If at all, it would result in maybe <5mins difference in runtime, with the screen running all the time. Any actual cpu work would have a far bigger impact.

If you really want to save power that way, choose a high-contrast theme that allows viewing the screen with lower backlight.

I would agree. Since to make black, an LCD has a backlight then applies current to all the LCD pixels to block the backlight and produce black, (rather than their usual transparent state)

It does get rather complicated though when you consider some LCDs automatically dim the back light when showing dark images to artificially increase their contrast ratio. And others are black when no voltage is applied to the pixels, and go clear when voltage is applied (eg VA LCDs). Seems like the N900 doesn't fall into one of these catagories though.

So basic advice would be use a high contrast theme so you can turn down your backlight which uses most of the LCDs power anyway.