This article inspired a quick search and I find this and a decent reference site. With summer, gas price sensitivity and energy in the news, the PR crowd is busy pointing to their sensible activities. Green roofs do not make sense everywhere. You need the right climate and the right roof. But they are a good idea when those match. As with APUs, this is not of interest to the homeowner or consumer. This is a commercial issue. But reducing commercial building costs and improving the city micro-climate are good things to do.
As with other no regrets policies, the green roof can be constructed and maintained for about the same cost as a regular roof. It saves some money on cooling, and may save money on waste-water management. It certainly improves the local micro-climate by reducing the summer heating effect from the building roof. It can improve the local drainage situation by controlling runoff . If done properly, it reduces the maintenance needs of the roof (which covers the slightly higher cost), because the wild temperature swings of a traditional roof cause insulation and other breakdowns in the roof. It may even save money on construction if it can substitute for other waste-water management construction.
In some situations, like offices and apartments, it can become an amenity as a rooftop garden, and it is often a selling point to the local community.
Green roofs do not make sense where the climate is too dry, or too wet, or too hot, or too cold. They make sense in the temperate climates, like Europe and the Eastern US. They might make sense in California. Areas like the LA basin suffer significantly from the micro-climate effects of the huge black expanses of the building roofs. I'm unsure about their water situation. And outside the core city you need to worry about fire. A green roof is a significant fire hazard when the plants dry out. LA might do better to concentrate on shade mechanisms and bleach white tiling for roofs.