Here are a couple of interesting documents that illustrate some of the thinking that went into the development of the NASA Centennial Challenges.
The first is the 2004 Centennial Challenges Workshop Report that shows a lot of the discussion and brainstorming that went into narrowing down the numerous proposals for prizes for the program. Obviously there is no shortage of problems to solve with the prize mechanism.
The second is a paper by Ken Davidian that covers the history of innovation prizes, and the history and state at the time of writing of Centennial Challenges. He uses a lot of the same sources that I used for the paper that eventually resulted in my starting this blog. I probably would have used this paper as a reference, too, if I'd encountered it then.