I'm not sure of the source for your grey water, but here in California a number of cities do make use of "recycled water" which is sewage that has been treated by a sewage treatment plant. It is not allowed for drinking but for other tasks, especially irrigation.

I don't know if it's used in many residential areas, but I know a lot of businesses around here use it for watering their landscape. It's also used in agrigultural operations.

I think the real problem as you mention is infrastructure. You have to keep the lines separate (one for recycled water, the other for drinking water) and how many home builders want to install two sets of pipes in a house, even assuming there's a recycled water line near them?

Stephen