Fysiks Wurks found on the U.S. Department of Energy website news of a breakthrough in solar energy efficiency
. From the article: "...with DOE funding, a concentrator solar cell produced by Boeing-Spectrolab has recently achieved a world-record conversion efficiency of 40.7 percent, establishing a new milestone in sunlight-to-electricity performance."
A page linked from Wikipedia's article on solar energy
calculates the land area that would need to be covered by solar collectors
at 8% efficiency to meet the world's energy needs (using 2003 figures). At 40% efficiency, it looks like a square 265 miles on a side in the American southwest would do it.