Well, I've been reading Jaron Lanier's recent book, You Are Not a Gadget, which gives a good reply (p. 102):
It is a common assertion that if you copy a digital music file, you haven't destroyed the original, so nothing was stolen. The same thing could be said if you hacked into a bank and just added money to your online account. ... The problem in each case is not that you stole from a specific person but that you undermined the artificial scarcities that allow the economy to function. In the same way, creative expression on the internet will benefit from a social contract that imposes a modest degree of artificial scarcity on information. [bold added]
I've made a similar point before: Money is information -- specifically, the relative amount that the world (believes that it) owes you. When money is stolen from you, then you can certainly force yourself to think of it in terms of
-a physical item being removed from your possession, or of
-a server having "unauthorized use".
But what's really important is the editing of that information: where before, the world thought it had a remaining balance against you of $X, now it thinks that the thieves are owed that $X. This problem persists even after you are given compensatory paper or the bank gets standard compensation for trespassing, and it's what people care about.
MMORPGs (World of Warcraft, Everquest, etc.) have already assimilated this lesson. In such online games, your money really is nothing but a database entry. It doesn't correspond, even in principle, to a physical object, just the knowledge of a relationship.
Lanier's alternate suggestion, following Ted Nelson, is that we could instead simply have an automated system that charges for each time a given intellectual work is accessed. People could "pirate" these (already freely-accessible) works by only using versions stored outside of where there access would be recorded, just as they do today when pirating works. But so long as the public regards this as wrong, and wrong for the same reason as counterfeiting, they would run into the same problem as counterfeiters. And the relatively low cost with which the works could be accessed under such a system would remove most of the sympathy for them.
I note that one particular snag of this is that people will not want to have to think of the costs each time they want to look at a book again. However, if everyone paid a fixed amount each year, and their choice of what to access only determined which fraction of that payment went to each creator, then choosing to view anything would cost nothing on the margin, further eroding any incentive to pirate.