Chelsea Green has an article this morning about using air-source vs. ground-source heat pumps. It's all about your climate, apparently. For instance, in Texas, you'd want to use an air-source heat pump in Houston and Corpus Christi and a ground-source unit in Lubbock and Wichita Falls. Add this to your Rules of Thumb list.
Chelsea Green » Blog Archive » Natural Home Heating: Air-Source vs. Ground-Source Heat Pumps