This is an oldish study, but we believe that it deserves all of the attention that it can get. The study analyses the economic impact to CD sales of music downloads. The study confirms what many of us music downloaders already knew (I hope that the BPI doesn't sue me). Music sales are not greatly affected by music downloads because most of the people who are downloading would not have purchased the CD anyway, and many who download music go ahead and buy the CD.
The study claims that "We find that file sharing has only had a limited effect on record sales [...] Even in the most pessimistic specification, five thousand downloads are needed to displace a single album sale."