According to a recent news article from the Green Wombat a scientist, David Mills presented a research paper at the Solar Energy Society conference which argues that large solar farms can replace traditional dirty power plants and produce the same electricity at competitive prices. He argues that by using solar thermal plants, combined with some sort of energy storage system to keep the lights on all night, we can totally replace dirty electrical production plants here in the US.
Solar thermal plants use mirrors to focus the sunlight (or electrical light) towards tubes of water which create steam to turn turbines. The idea of using electrically generated light at night is what sets his idea apart and actually makes it feasible on such a large scale. By using this technology Mills calculated that a 92 mile square solar farm could power the entire continental United States.
To read more have a look at the full article over at Green Wombat. http://greenwombat.blogs.fortune.cnn.com/2007/09/19/solar-nation-can-the-sun-power-the-us-3/