Technologist David Reed once said, “There's no scarcity of spectrum any more than there's a scarcity of the color green."  This quip presumes the technically correct meaning of spectrum as a range of vibration frequencies of electromagnetic waves. However, it’s clear that when most people talk about spectrum, they don’t mean a vibration frequency. What do they mean?
I've been working through examples of how lay people conceive of spectrum, and talk about spectrum policy. An early draft (Word doc; source data in spreadsheet form) explains why some policies and proposals make sense to us, and other things don’t. I’m not making claims about how experts think, though I suspect that the metaphors I’ll describe is at the root of their thinking, too.
In summary, spectrum is conceived of as a spatial resource, with two common variants: spectrum as a set of containers (bands), and spectrum as land. There are two common mental models of wireless signals: as objects moving through space, and as sounds, particularly speech. This leads to two mental models for interference, which entail conflicting, and sometimes incorrect, deductions.
Spectrum, as the concept is treated by regulators and politicians, is a resource used for communication which is, in the first instance, under state control. Its assignment is thus the stuff of politics, that is, arguments over the distribution of scarce resources. The spectrum-as-land model is “natural” to most people because the underlying spatial metaphor, of real estate in particular, fits our notion of land resources.
The results of this analysis can be used to identify policy-making pitfalls. For example, Hatfield & Weiser  explain why the transition to a property rights model for spectrum is far more complex than commonly portrayed; this work hopes to explain why a model of real property rights is attractive in the first place.
 Quoted by David Weinberger in The myth of interference, Salon 12 March 2003. Curiously, if one considers electromagnetic radiation in optical fiber, there is indeed “a scarcity of the color green” because each fiber supports a finite number of wavelengths. For example, links in the National LambdaRail network use dense wavelength-division multiplexing (DWDM), which allows up to 32 or 40 individual optical wavelengths to be used (depending on hardware configuration at each end). Once those wavelengths are occupied, no more are available.
 Hatfield, Dale N and Philip J Weiser (2006), “Property Rights in Spectrum: Taking the Next Step,” University of Colorado Law School, Paper Number 06-20, June 2006, http://ssrn.com/abstract=818624