Heap Media has responded to Mark Ontkush's observation that a black background version of Google would save 3000 Megawatt-hours a year by going ahead and building one.
From a usability perspective, white type on black is clearly harder to read, so I don't think this is a final design answer. But I like how Blackle sparks conversation about new and better ways to green the web. Kudos.
(Thanks to Todd for showing me this.)