The test-of-concept model could be totally paper-based -- a sketch treatment, indicating program structure and the sort of features (interactive and otherwise) to be included. Personally, I favour building interactive sketch models (draft applications) and, for this, I find Hypercard an ideal program. With Hypercard on the Mac (or other similar authoring programs), interactive models can be quickly constructed in a form that is easily adjusted and gives more concrete expression to the way those involved in the initial design process are thinking. In this way, decisions about what works are easier to make. Working in this way can also make later phases like content-writing a lot easier.