Travis asked about references I used in my design. I decided, that putting my response in a comment was going to be a bit limiting, so here goes...
I have three books that are specific to underground house design. Then I have a number of other books about building your own house, working with green roofs, designing radiant heat system, designing solar heat systems, designing solar electric systems, residential wiring, plumbing, code books...yes, I am going to have quite a collection for the "building my house" bookshelf when I get done.
The three that I have that specifically target underground house construction are:
Underground Homes by Lois Wampler - I think this provides a nice overview of a lot of topics relevant to earth sheltered house construction
Earth Sheltered Housing Design guidlines, examples, and references by the underground space center of minnesota - it is a very nice reference on the trade-offs of different arrangements of insulation, earth sheltering, and other construction considerations such as waterproofing and drainage, and loading of walls. It has a number of examples of different constructions, and how they compare on energy usage.
Earth-Sheltered Houses by Rob Roy - In my opinion, this provides a very thorough discussion of pretty much all the critical aspects of earth sheltered house construction. They have a couple designs in there, that I think I would have been quite happy with, except for some of the restrictions that I have based on my location. There are a couple things that they outline, that I think I would have been happier with, if I could have used them.
I also have done quite a bit of searching on the web for blogs, message boards, and pages dedicated to information about earth sheltered/underground home construction. I am hesitant to post links, as some of them have already gone away since I found them, but I will give you these, and hopefully you will find them to be of use:
Tom's PAHS - http://paccs.fugadeideas.org/tom/index.shtml
EERE Consumer's Guide: Earth-Sheltered Home Design - http://www.eere.energy.gov/consumer/your_home/designing_remodeling/index.cfm/mytopic=10100
Earth Sheltered Homes - Rocky Mountain Research Center - http://www.earthshelters.com/
Phil Malone and Lisa Malone's Earth Sheltered, Geothermal, Passive Solar home at Deep Creek Lake, Maryland. - http://www.ourcoolhouse.com/
Best of luck to you Travis.