From the RIAA/MPAA statement today:

A September 2006 study commissioned by the Institute for Policy Innovation sought to calculate the costs of motion picture piracy, not only on the specific copyright owners whose works were subject to infringement, but also on upstream suppliers, downstream purchasers, and the cascading effects on lost output, lost earnings, lost jobs, and lost tax revenues. It concluded that the estimated $6.1 billion lost by MPAA member studios in 2005 translated into economywide total lost economic output of $20.5 billion.

From the GAO statement yesterday:

Another example of the use of surveys is the study by the Motion Picture Association, which relied on a consumer survey conducted in several countries. This study found that U.S. motion picture studios lost $6.1 billion to piracy in 2005. It is difficult, based on the information provided in the study, to determine how the authors handled key assumptions such as substitution rates and extrapolation from the survey sample to the broader population.