For massive blockbusters they do find a negative effect, the positive impact is mainly for lesser-publicized movies without the massive advertising budget funneled into them.
It's always hard to tell with statistics, but I don't think it's a stretch to suggest that the extra publicity from file sharing outweighs the negative effects for those smaller movies.
I imagine that most of the people who would want to see a blockbuster will know about it due to the overwhelming marketing. At the other end of the marketing budget spectrum the movies probably rely on word of mouth which would probably get a boost from file-sharing.
Even so I want to believe the story, it smells a lot like the typical correlation = causation story. These things are so easy to construct, i.e. the pirates & global warming correlation [1].
There could be a lot of other factors that were leading to this development, e.g. studios concentrating more on pushing blockbuster movies, generally a weak indie/strong blockbuster movie season, etc.
