Some recent events have caused me to become interested again in the concept of intellectual property. I've often thought of this as somewhat of a fringe issue, interesting and demanding further attention, but among the lesser pressing problems of the day. However, this view is something I'm coming to reconsider, and either way I think it is at least an interesting question.
It helps to begin with our conceptions of ordinary property. John Locke's idea of property was that if you find something in nature, and "add/mix" your labor with it, you thereby gain property rights over this thing. (The actual theory is obviously much more involved, I'm just going for the philosophy 101 snapshot right now.) So if you take some wood and you build a hut with it, that's your hut. If you pick some apples, those are your apples, and if you develop some land into farmland, that's your farmland. Once people own things, fair and legitimate trading can commence.
There are many well-known problems with this theory, not the least of which is that it derived some of its appeal as a justification for taking the lands of more nomadic peoples. In a globalized world, in which we face resource shortages and overcrowding of certain areas and over-consumption, as well as incredible inequality, I think Locke's natural law theory of property rights is less plausible than it once seemed. I think more promising theories of property rights see these rights as arising out of the social contract, as a pragmatic solution to the distribution and possession of goods rather than the consequences of some metaphysical relationship we have with objects.
Now there's obviously a lot more that could be said about property in those terms, and that's a very interesting discussion in itself, but this should suffice for now. Having a conception of property rights, understood as a pragmatic agreement about how certain goods may be come reserved for an individual's use and benefit under protection of the law, we can see why stealing is wrong. If you steal someone's property, you (1) undermine the social arrangement that is useful for all of us, you (2) deprive them of something that they've fairly obtained through a process developed by social convention and impose a cost upon them, and you (2) unfairly benefit from their deprivation.
All of which assumes, of course, that underlying framework of legal property ownership is just, which is a huge assumption that is definitely open to challenge at present. But even if you think that the underlying social conventions of property rights are unjust, this does not imply that there are no moral reasons not steal. It likely remains true that a system of unfair property law is better than a society without any property laws, so the reasons against stealing still apply (thought perhaps to a limited extent). We have reason to support the social contract that we have in place, even if it's not ideal.
Now consider the case of theft of intellectual property. I've seen many ads and people who claim that "You wouldn't steal a purse, so you shouldn't illegally download a song." But I think the reason people feel the need to make this analogy is that the rhetoric behind intellectual property rights supports such an analogy, despite the fact that intuitively, people think there is a difference. The problem with the rhetoric is that it doesn't answer the question someone might seriously ask, which is what makes theft of intellectual property wrong?
The concept of intellectual property, it seems to me, is not nearly as important to social cohesion other kinds of conceptions of property. We know this because many societies have gotten along fine without intellectual property law as we know it. Also, many cases of intellectual property "theft" (i.e. illegally downloading a song) don't actually deprive the victim of anything. It's true they don't get the income they'd get if you paid for the use of their intellectual property, but this also is the case if you never hear their music. It's not the same as someone actually being materially worse off when you steal their purse, because when you steal someone's purse, they end up with fewer resources than before, while you end up with more. If you steal somebody's intellectual property, you are benefited (you have a new song to listen to!) and they haven't lost anything.
Of course, the reason we have intellectual property laws is that it is, supposedly, a useful part of the social contract. The idea is that the fact that people are able to have intellectual property rights makes it more profitable for them to produce there intellectual property, and thus they are ale to produce more and better material. So if it is true that the property laws work this way, intellectual property "theft" would likely be objectionable, because it undermines a beneficial social arrangement, and unfairly benefits the thief.
Notice how, even assuming the strongest case for intellectual property law, the objection is not as strong as the objection to the case for material theft. Obviously any two cases would differ on a wide range of specific considerations, but in principle there is an important disanalogy between property theft and intellectual property theft, and property theft is in principle worse. This is because the "victim" of the theft is not left off materially worse than they would be had the theft not occurred.
And the question of whether or not intellectual property law is on the whole beneficial to society is an open one. The answer would have to consider many empirical facts that are not my forte, and on which I am not sufficiently informed. But if it's true that intellectual property law does not bring a net benefit to society, and we would be better off without it, the remaining reasons that I have given to object to intellectual property theft evaporate..