Martin Fowler just added an article on the DesignStaminaHypothesis to his bliki (a cross between a blog and a wiki) to answer this question. His hypothesis is that design will slow the initial creation of functionality, but at some point ( the "design payoff line"), you will be able to add new functionality more quickly by leveraging design. I state this as a hypothesis because he does also. While it seems perfectly intuitive to any practicing software developer, and most software developers accept it as true, it cannot really be proven since there are so many aspects of software development that are difficult to measure (output, quality, etc.)
I definitely agree with this judgement that the design payoff line is lower than most people think, and that design is critical in creating software that will both work well today and enable changes tomorrow.