A problem we have in Toronto with green roofs is that so many of our nicest houses have steep pitched roofs which basically precludes their greening. I suppose you could slap on some solar panels though. There is going to be a clash between people mindlessly trying to protect older architecture and the people trying to create sustainable housing from our existing stock. I think it would be a great project to see how green you can make an old toronto victorian gable roofed house, while maintaining the original envelope and over all look. It would take alot of money though. I find prefabs to be a little boring and lacking in character personally.

heres a nice national geographic article on low budget greening:
http://www.thegreenguide.com/doc/108/ecohomegreenhome