I love this as a guest house. Owen Geiger has this picture tutorial over on Instructables showing us how to build an earthbag roundhouse. Very economical to build and he has it complete with plumbing and electrical wiring.
I would love to have one of these in my back yard as an office. It could be used for so many things. Guest house,office,studio, small rental for grown children or older parents. As the economy seems to just keep getting worse I think we will have to be going back to multi generational family living and this would be a neat way to have more people helping with all the bills but giving everyone some of their own space.