Mother Earth News has a great article (Oct/Nov 2006) about Earth-sheltered homes, including tips on building techniques, living roofs, and how earth-sheltering works in general. Definitely intended for the construction- or engineering-minded, but well worth the read.
Longevity. The properly designed earth roof is the longest-lasting roof you can build, because the earth and insulation protect the waterproofing membrane from the three conditions that eventually break down every other common roof surface: ultraviolet radiation, erosion and freeze-thaw cycling, which all damage exposed roofing over time.
Insulation. Solid earth is not good insulation, but 3 or 4 inches of light soil with vegetation growing in it has some insulation value. In winter, the cold, uneven earth roof holds snow better than other roofs, and fluffy snow is a good — and free — insulator."
Link to article.