There is no way Brokeback 'usurped' Walk the Line. It was Munich that knocked off Walk the Line. The thing is that with the 'preferential ballot' system used by the Oscars, a film has to have a lot of first and second place votes to get nominated. Walk the Line could've been 2nd and 3rd on everyone's ballot, but still missed out because it didn't have enough first place votes.
Is there is listing somewhere of the votes etc?