700MHz is particularly valuable spectrum because it passes through solid barriers more easily than other frequencies. The New York Times has a good background article:
The radio spectrum licenses, which are to be returned from television broadcasters as they complete their conversion from analog to digital signals in February 2009, are as coveted as oil reserves are to energy companies. They will provide the winners with access to some of the best remaining spectrum — enabling them to send signals farther from a cell tower with far less power, through dense walls in cities, and over wider territories in rural areas that are now underserved.
The links to the right will take you to the FCC site for this auction, and to Wired Magazine’s FAQ.