Have they ruled out the possibility that the people marketing the movie are gaming the system? Even if voting was restricted to established IMDB users, surely a reasonably well connected Hollywood marketing person could round up a few hundred of those among friends and friends of friends.
Hmm, those numbers are definitely too big to be influenced by anyone's network. Judging from the other top-ranked films, it looks as if IMDB's userbase is in the middle of changing.
I don't think they need to game the system. The lukewarm or negative reviews on RottenTomatoes are being bombarded with genuine fanboy vitriol. (Many of the commenters, at least on the first page of comments, have joining dates that suggest they have been on RT for a while)
http://www.rottentomatoes.com/m/the_dark_knight/?critic=crea...
It is quite amusing in a sad way actually, seeing them trash the likes of David Denby and Joe Morgenstern.
IMDB's ranking system is one of the least sophisticated out there, which is surprising given that they're owned by Amazon these days. I assume it's still just the same couple of guys that started it left to run it in relative independence. You really don't need to game their system much; it's just bad technology.
