As mentioned above, CRTs see (some) power saving when more pixels are black. Same goes for plasma and OLED screens.

Some LCDs (typically TVs and some computer monitors) do have local backlight control, such that they can dim the backlight in a dark region of the screen -- while this is targeted mainly at enhanced dynamic range, it also saves some power, but it's per-sector, not per-pixel, so it's not clear how much difference a white-on-black vs. black-on-white theme would make.

Most LCDs, including the ones in the N900, have a single backlight controlled by a single brightness control, which directly affects power draw. While the display matrix power consumption may be better with white or black pixels, the total display matrix power is significantly less than the backlight power, so IMO this isn't worth worrying about at all -- better to pick a theme that maximizes readability so you can dial the brightness down a notch.