Singapore's EDITT Tower is one of the coolest building designs I've seen in a long time: it essentially takes the green roof concept and morphs it, by greening not just the roof, but the sides and interior of a giant 26-story tower. The idea is to also integrate the upper floors with the streetscape by providing a continuous series of ramps and greenways inside and outside the building -- one third of the space, overall, is being used for greenery, water catchment (half the water used by the inhabitants will be supplied from natural rainfall), and the like. Of course there're solar energy and green building elements.
The 'Huggies have a great overview, if you want more -- but we'll be coming back to this one!
Sounds like they're riffing on Hundertwasser's "tree tenant" notion.
The "green roof" link is broken (empty).