Felix Oberholzer-Gee and Koleman Strumpf of the Harvard Business School have released "File-Sharing and Copyright" as a working paper.
Here's an excerpt:
As our survey indicates, the empirical evidence on sales displacement is mixed. While some studies find evidence of a substitution effect, other findings, in particular the papers using actual file-sharing data, suggest that piracy and music sales are largely unrelated. In contrast, there is clear evidence that income from complements has risen in recent years. For example, concert sales have increased more than music sales have fallen. Similarly, a fraction of consumer electronics purchases and internet-related expenditures are due to file sharing. Unfortunately, we know little about the distribution of these impacts. How markets for complimentary goods have responded to file sharing remains an area of inquiry that is largely unexplored in academic research.