I had a friend over this weekend that was a huge NBA nut and we got talking about Tim Donaghy. Besides Tim affecting the games he reffed, he also gave out the referee assignments before they were officially announced. (The NBA is not pursuing this avenue because they don't want the bias in referees to be in the lime light). The mob could then bet on the games before do to the tendencies of the referee.
I looked and saw what the difference spreads were in baseball and here are the hitter and pitcher friendly umpires against the spread in 2008 (at least 28 games behind the plate):