One Silas Barta keeps claiming that those who support property rights in airwaves cannot object to property rights in IP (see here, here, here, here, and here). My response to this argument, in brief, is: (a) so what? If this were so, then that just means we cannot support property rights in airwaves; (b) and I disagree with this since the airwaves are scarce resources; (c) the libertarian case for property in airwaves is not settled or very developed yet.
As to (c), again, I refer to Kelley and Donway's monograph. As I have maintained for years, I lean in the direction indicated by Kelley and Donway--that individuals could on the free market homestead EM spectra. Let me briefly outline how I think this could work in a free market. First, let me note the argument of those who object on the grounds that EM frequencies are mere numbers. But a given "airwave" is basically a bandwidth of radio frequencies over a given limited volume of the earth's surface. Radio waves are electromagnetic waves that propagate at the speed of light (they are light; visible light is just one portion of the EM spectrum) through space. A signal can be transmitted by sending a modulated signal at a given wavelength (wavelength is inverse to frequency). For example AM means amplitude modulation: a given signal is modulated by varying its amplitude (magnitude). In FM, the frequency is modulated. It is impossible to have a perfectly precise (narrow) wavelength; when you emit a signal it is at a "center" wavelenght but extends to the "sides" to some degree, called the bandwidth. So a given broadcaster might send a radio signal over a certain bandwidth (set of wavelengths) surrounding a center wavelength or frequency such as 87.5MHz.
Now the case for property in airwaves is basically this, as I see it. It is based on the basic idea of homesteading (see my What Libertarianism Is). Under this approach, every scarce resource--things that can be contested; rivalrous things; resources that have exclusive use, so that use by me excludes use by you, and so on--is assigned an owner; that owner is the person who first appropriated or used the property in an embordering way--that is as an owner. It's the first person to erect publicly visible boundaries that others can respect and see; he has a better claim to the resource than any latecomer.
Note that this approach implies that the type of use made of and the type of borders established for a given resource determine the scope and nature of property rights. Property rights in bodies are different than property rights in land and in cows and in an apple. One type of property right is an easement (servitude, in the civil law). Say people routinely walk over a path from point A (their village) to point B (watering hole). You can say they have homesteaded at least that use of the propety. If someone else builds a home there, they have to let the easement continue. A similar case can be made for airways ("tunnels" in the sky that airplanes pass thru) and shipping lanes.
Now, why does it have to be an easement over walking-space on land on the earth's surface, established by the passage of human bodies? Why can't it be an easement over the airwaves (viewed as a type of volume of space with respect to a given bandwidth), established by the passages of EM waves? After all, EM waves are physical, and the airwave/spectrum used is a scarce resource like a path on land is. It's scarce because broadcaster 1 can't use the spectrum if broadcaster 2 sends a transmission over the same carrier channel in the same physical region; there is interference. So: the idea is that the first broadcaster to use a given bandwidth in a given region would be regarded as having homesteaded this airwave or EM spectrum. Others who broadcast on it would be viewed as trespassers. [Incidentally, the reason transmission of an EM wave for information-signalling purposes is not normally trespass even though the waves pass through others' bodies is that it does not interfere with their use of their bodies; for more on this approach to "invasion" see Rothbard's classic air pollution article; by contrast, aiming high-intensity, coherent, power-carrying EM radiation--a laser beam--at someone could be aggression, since it could affect the physical integrity of their body or other property.]
I tentatively lean in favor of this argument. I suspect this is the type of argument, and practical use, that would tend to win out in a decentralized, property-respecting, dispute-settling society. But I am not 100% sure.
A final note. This type of situation is not analogous to IP because there is no "idea space" that is scarce and homesteadable. Instead the IP advocates want to install property rights in "the right to make a greater profit by virtue of artificial imposed scarcity," something like this.
A few other discussions of this can be found here: B.K. Marcus, The Spectrum Should Be Private Property: The Economics, History, and Future of Wireless Technology; and comments here: http://blog.mises.org/archives/005577.asp#comment-101581; http://blog.mises.org/archives/006930.asp#comment-123893; and http://blog.mises.org/archives/007561.asp#comment-133579; http://blog.mises.org/archives/007561.asp.
Update: Rothbard was also, as usual, far ahead on this issue. Some of his comments on this are below:
From Man, Economy, and State:
Furthermore, if we understand by "air" the medium for the transmission of such things as radio waves and television images, there is only a limited quantity of wave lengths available for radio and for television purposes. This scarce factor is appropriable and ownable by man. In a free society, ownership of these channels would accrue to individuals just like that of land or animals: the first users obtain the property. The first user, Jones, of the wave length of 1,000 kilocycles, would be the absolute owner of this length for his wave area, and it will be his right to continue using it, to abandon it, to sell it, etc. Anyone else who set up a transmitter on the owner's wave length would be as guilty of invasion of another's property right as a trespasser on someone else's land or a thief of someone else's livestock.
... If a channel has to be a certain number of wave lengths in width in order to permit clear transmission, then the property would accrue to the first user, in terms of such width.
Professor Coase has demonstrated that Federal ownership of airwaves was arrogated, in the 1920's, not so much to alleviate a preceding "chaos," as to forestall this very acquisition of private property rights in air waves, which the courts were in the process of establishing according to common law principles. Ronald H. Coase, "The Federal Communications Commission," Journal of Law and Economics, October, 1959, pp. 5, 30-32.
It is rapidly becoming evident that air lanes for planes are becoming scarce and, in a free society, would be owned by first users--thus obviating a great many plane crashes.
The theory of homestead easements discussed earlier would require no restriction upon radio transmissions or on people's low-level radiation. In the case of radio transmissions, Smith's ownership of land and all of its appurtenances does not entitle him to own all radio waves passing over and across his land, for Smith has not homesteaded or transmitted on radio frequencies here. Hence, Jones, who transmits a wave on, say, 1200 kilohertz, homesteads the ownership of that wave as far as it travels, even if it travels across Smith's property. If Smith tries to interfere with or otherwise disrupt Jones's transmissions, he is guilty of interfering with Jones's just property.
Only if the radio transmissions are proven to be harmful to Smith's person beyond a reasonable doubt should Jones's activities be subject to injunction. The same type of argument, of course, applies to radiation transmissions.
...  During the 1920s, the courts were working out precisely such a system of homesteaded private property rights in airwave frequencies. It is because such a private property structure was evolving that Secretary of Commerce Hoover pushed through the Radio Act of 1927, nationalizing ownership of the airwaves. See Ronald H. Coase, "The Federal Communications Commission," Journal of Law and Economics 2 (October 1959): 1-40. For a modern study of how such frequencies could be allocated, see A. De Vany, et al., A Property System Approach to the Electromagnetic Spectrum (San Francisco: Cato Institute, 1980).
in the courts' determination of radio frequency ownership in the 1920s, the extent of ownership depended on the technological unit of the radio wave -- its width on the electromagnetic spectrum so that another wave would not interfere with the signal, and its length over space. The ownership of the frequency then was determined by width, length, and location. ... American land settlement is a history of grappling, often unsuccessfully, with the size of the homestead unit.
See also Toward Property Rights in Spectrum: The Difficult Policy Choices Ahead, by Dale Hatfield and Phil Weiser (Cato 2006); and Milton Mueller, Property Rights In Radio Communication: The Key to the Reform of Telecommunications Regulation (Cato 1982).