I had an interesting lunch recently with Michael Calabrese, who is a VP and director of the Wireless Future Program at the New America Foundation. He has been a champion of the notion that much of the radio spectrum is unused and that building smarter receivers and transmitters can unlock enormous amounts of spectrum which are over-allocated due to the need to protect "dumb" receivers. NAF advocates a commons approach to spectrum that might be freed up in this way; other organizations with the same technology approach take a spectrum-as-property approach. Nevertheless, to us broadcasters who consume lots of spectrum and to those of us in public radio who have run out of spectrum for pubic service, efforts in this regard should be of strong interest.
It turned out that Michael's program within NAF had just released four papers on this subject. I've now either read or scanned all four and suggest you checking them out at the links below. These links go to executive summaries of each paper, with a pdf link provided at the end of each one.
Michael Calabrese, The End of Spectrum Scarcity.
Preston F. Marshall, A Potential Alliance for World-Wide Dynamic Spectrum Access.
Michael J. Marcus, New Approaches to Private Sector Sharing of Federal Government Spectrum.
Sascha Meinrath and Victor Pickard, Revitalizing the Public Airwaves.
The NAF at which these papers were released featured these individuals plus Kevin Werbach of the Wharton School at the University of Pennsylvania and Tom Stroup, CEO of Shared Spectrum Company. You can listen to an audio recording of these event here.
Finally, Nate Anderson has an accessible article of this in Ars Technica, White spaces on steroids and free spectrum for everyone.