Looking further in the NASA budget document, I found that the Centennial Challenges program has been moved inside the "Innovative Partnerships Program" (IPP). On page 460, there is a table with the following entry for CC:
Project: Centennial Challenges
Commitment: Hold 3 Centennial Challenges competitions
Fy07 request: $10.0M
Fy08 request: $4.0M

The CC is described as follows (I won't block quote to save space):

"The Centennial Challenges program conducts prize competitions for revolutionary, breakthrough accomplishments that advance the Vision for Space Exploration and other NASA priorities. Some of NASA's most difficult technical challenges may require novel solutions from non-traditional sources of innovations. By making awards based on actual achievements, instead of proposals, NASA is tapping innovators in academia, industry and the public. This effort is modeled on successful past prize competitions, including an 18th century navigation prize, early 20th century aviation prizes, and more recent prizes offered by the US government and in the private sector.

"Despite the fact that none of the competitions held to date have resulted in the award of a cash purse to a winning team, the amount of team diversity (representing small and large businesses, high school and university students, and enthusiastic hobbyists and garage mechanics) and the variety of technologies implemented exceeded all expectations.

"As the prize purses increase, the amount of participation and level of technical maturity and ingenuity will also increase. In the past competitions with prize purses on the order of $300K each, it is estimated that the 10-15 participating teams represented an investment of $50-100K each. Teams invested on the order of $250-500K each in the competition with the $2M prize purse.

"With the FY 2008-2012 funding request of $4M/year for Centennial Challenges, new prize competitions will be initiated to support NASA's science (e.g., the Station-Keeping Solar Sail Challenge), aeronautic (e.g., the Micro Reentry Vehicle Challenge), and space exploration (e.g., the Human Lunar All Terrain Vehicle Challenge) goals. Centennial Challenges is continually working with each of the NASA Mission Directorates to ensure that competitions selected are addressing the current set of NASA's technology priorities."