Virtually Blind has an interesting article featuring a discussion paper on virtual property rights.
The paper is written by a Columbia Law School graduatem, Daniel Gould and you can read it here. It covers the challenges in defining virtual property and the part that stood out for me was:
bits in memory do not, by themselves, possess the qualities of real
world property mentioned above. Rather, it is the bits in the context of the MMOG
application interpreting them that simulates the features of physical property.
If you like the intellectual stimulation of bits, in our out of context, give this paper a read.