Today, significant money and political capital are being expended to obtain and hold onto usable license-exempt access in the TV white spaces. These efforts are important for applications today but there’s a follow-on spectrum initiative that, if successful, would yield much greater benefits in the long term. We should be seeking similar access to as much as possible of the spectrum above 3 GHz, almost all of which is dramatically under-utilized today.
Throughout the 20th century and right up to today, it's been the case that higher frequencies "don't go as far." But this is the result of technology limits, not ultimate physical limits, and these technology limits are now being overcome.
Within ten years it will be widely apparent that higher frequencies go just as far through the atmosphere, they do just as well at penetrating buildings, and they have other extremely important benefits that lower frequencies lack.
Among their many advantages, directional antennas are smaller and more economical at higher frequencies. Directional antennas reduce received interference and facilitate spatial reuse, thus vastly increasing the utility of higher frequency spectrum.
What’s more, it's easier to send high-speed data because there is more spectrum available at higher frequencies. We'll never be able to send 1 Gbps over any real-world 6 MHz TV channel but, above 3 GHz, we can easily find 200 MHz of spectrum that's temporarily vacant and that's enough to carry more than 1 Gbps of data even with today's technology.
For the next decade or two, TV white spaces will continue to be important for penetrating foliage, but even with foliage, the physics of what is possible differs from 20th century experience. In the future, the real action will be above 3 GHz.
Finally, while it's never easy to persuade existing licensees to accept secondary users in “their spectrum” even while it’s idle and they are non-interfering, it should be easier to fight the political battles now, when most people don't realize the long term value of spectrum above 3 GHz. Now is the time we should be seeking license-exempt access to as much as possible of the white spaces above 3 GHz.
Details for the technically inclined
All photons (light or radio waves of any frequency) go at the same speed (the "speed of light"). In our atmosphere, photons at frequencies above 10 GHz are subject absorption because they excite resonances in atmospheric molecules like water vapor or oxygen. But the atmosphere is transparent to radio signals between 30 MHz and 10 GHz so, with a clear line-of-sight, radio signals at 8 GHz go just as far as signals at 700 MHz or 50 MHz .
This physical fact is sometimes missed because the Free Space Path-Loss (FSPL) equation (see: http://en.wikipedia.org/wiki/Free-space_path_loss) commonly used to calculate radio frequency (RF) transmission losses actually encapsulates two effects. These are 1) the actual path loss (which is independent of frequency) and 2) the receiving antenna aperture which is based on wavelength. Thus the FSPL equation assumes smaller antennas for higher frequencies and of course, smaller antennas collect less energy. With equal antenna apertures, unobstructed line-of-sight radio transmissions are frequency independent, even with 20th century technology.
The problems that have favored lower frequencies are reflection, refraction, polarization and diffraction. Higher frequencies have shorter wavelengths and shorter wavelengths signals are more easily scattered. Scattered signals that reach the receiver have taken a longer path and thus arrive a little later. With 20th century technology, these delayed signals (called "multi-path" signals) were just part of the noise degrading the primary signal. Now with Multiple Input Multiple Output (MIMO, see: http://en.wikipedia.org/wiki/MIMO), it's possible to decode multi-path signals, remove them from the noise, align them in time and add them to the primary signal - multi-path signals are no longer a deficit but actually improve system performance!
MIMO technology only began to emerge in the mid-1990s but it is now an option in the latest Wi-Fi, WiMAX and LTE specifications. MIMO uses multiple radios and higher order MIMO requires increasingly sophisticated calculations, but early (2x2) MIMO systems are already widely deployed in 802.11n consumer WiFi products and continued semi-conductor progress (following Moore's Law) will make MIMO calculations and additional radios ever lower cost.
Also inherent in higher order MIMO is beamforming and beamsteering. As the number of radios and antennas in a MIMO system increases, the system is able to simulate tighter and tighter beams, providing ever more spatial reuse of spectrum and more range or capacity for individual connections. However, tighter beams require more wavelengths of separation between the outer most antenna elements. Again higher frequencies have shorter wavelengths, so antennas that support tighter beams require less space at higher frequencies. For example, a 10 degree beam at 700 MHz requires an antenna ~10 feet across. To do the same at 8 GHz, the antenna need only be ~10 inches across.
Long term, the spectrum above 3 GHz will be more valuable than the spectrum below 3 GHz. Let’s get license-exempt access to these white spaces now, while the political stakes are still (relatively) low.
Some useful references
 Spectrum occupancy measurements for frequencies below 3 GHz, see: http://www.sharedspectrum.com/papers/spectrum-reports/. Fewer measurements have been made above 3 GHz but those done show negligible occupancy (certainly when compared with bands below 3 GHz), for example see: http://www.sharedspectrum.com/papers/spectrum-reports/
 This graphic http://en.wikipedia.org/wiki/File:Atmospheric_electromagnetic_opacity.svg shows the transparency / opacity of our atmosphere at different wavelengths. Note that 10 meters is 30 MHz and 3 cm is 10 GHz.
 While the total atmosphere (as seen from space) is highly absorbing above 10 GHz, shorter range point-to-point connections are still possible as this more detailed spectrum shows for the range 10 GHz - 1000 GHz: http://www.omlinc.com/library/other-references/atmospheric-absorption-of-millimeter-waves.html
 NIST, Electromagnetic Signal Attenuation in Construction Materials, NISTIR 6055, http://fire.nist.gov/bfrlpubs/build97/PDF/b97123.pdf Note that ordinary window glass is essentially transparent to RF at the frequencies tested (500 MHz to 8 GHz) while most other building materials provide substantial attenuation. One caveat: as this study was done to help design RF-based measurement device for the construction industry, they post-processed their data to remove delayed signals. In other words, MIMO communications systems will do considerably better than these measurements suggest.
 Okamoto, Kitao & Ichitsubo, Outdoor to Indoor Propagation Loss Prediction in 800 Mhz to 8 GHz Band for an Urban Area, March 2009, http://ds.lib.kyutech.ac.jp/dspace/bitstream/10228/2399/1/IEEE.pdf Detailed measurements demonstrating frequency independence for signals penetrating real life buildings.
 Perras & Bouchard, Fading Characteristics of RF Signals due to Foliage in Frequency Bands from 2 to 60 GHz, http://horwitzinternational.com/PDF%20Files/Trees%20and%20801_11.pdf Careful measurements show signal attenuation peaks when RF wavelengths equal leaf size and then falls off at even higher frequencies.
 RF Engineering for Wireless Networks: Hardware, Antennas and Propagation, by Daniel M. Dobkin, PhD, ISBN 0750678739 has useful chapters on signal propagation in the atmosphere, in the environment and in buildings.