Seems to me that downloading and box-office slumps are unrelated phenomena. In other words, the recent bad box-office years have been driven by crummy movies, not piracy.
Link (Thanks, Butch!)
Update: Stephen sez, "Back in November 2006 when the preliminary numbers came out, I wrote about the sales increase and some of the MPAAs anti-piracy propaganda, in which they determine that they lost $18.2 billion due to piracy in 2005. Fully 200% of their box office total *on a growth year*. How did they arrive at this number? They say only this: 'Piracy loss calculations are based on the number of legitimate movies - movie tickets and legitimate DVDs - consumers would have purchased if pirated versions were not available.' Very scientific!"