Speaking of Ryan Singer, I recently re-read his answer on the Quora startup thread Should I focus on a good user experience, or push something out quickly? He makes a really good argument for investing in design very early in the product development process:

Design is a path-dependent process. That means the early moves constrain the later moves. On the very first iteration the design possibilities are wide open. The designer defines some screens and workflows and then the programmer builds those. On the next iteration, it’s not wide open anymore. The new design has to fit into the existing design, and the new code needs to fit into the existing code. Old code can be changed, but you don’t want to scrap everything. There is a pressure to keep moving with what is already there.

Our early design decisions are like bets whose outcome we will have to live with iteration after iteration. Since that’s the case, there is a strong incentive to be sure about our early bets. In other words, we want to reduce uncertainty on the first iterations.

This is why variation, not just iteration, is so important during the early phases of a product.