As far as television is concerned most of the 'unused spectrum' relates to what is left over in the shift from analog TV to the more spectrum-efficient digital form. There are also significant chunks being used inefficiently by emergency services and other legacy uses. The key is to incentivize this type of user to update to more spectrum-efficient utilization. In the case of the TV broadcasters, there is also the problem that they have come to expect that they own that spectrum and they have other ideas on how to use it. All of this is going to be a significant challenge for the FCC over the next few years. I really hope that they are up to it.
Exactly what constitutes "a broadcaster with a piece of unused or underused spectrum"? From one perspective, a TV broadcaster is fully using his spectrum if he is transmitting a 19.39 Mbps ATSC signal on his licensed channel. But from the other perspective, if only 13% of U.S. TV households actually use an antenna to receive that ATSC signal, then is it not true that all TV broadcast spectrum is underused?
Are these voluntary incentive auctions just another step toward the elimination of free over the air television in the U.S.?