Business Week has an article on the X PRIZE Foundation's goals for its Energy & Environment X PRIZE Suite. The plans for the Biofuels Prize, the next one planned after the Progressive Automotive X PRIZE, are described:
... foundation President Tom Vander Ark said the biofuels prize would be launched late this year, followed by the other categories over the coming two years. The biofuels prize will be worth at least $10 million, he said. ... Among the rules are that contestants must produce a biofuels plant that uses nonagricultural fuel. The plant must also be small-scale and easily shipped, thus making the technology applicable anywhere in the world.
There are also more hints about later energy and environment prizes, with a goal of $100M in prizes:
He says one prize will be for innovation in providing water, broadband, and clean electricity to villages in the developing world. Other energy categories will be for innovation in energy transmission and the construction of energy-efficient houses and commercial facilities.