Yahoo Sports columnist Dan Wetzel had an interesting column yesterday about the beating that Vegas sports books are taking on NFL games this season. The "standard" model of sportsbook behavior predicts that point spreads are set to attract an equal volume of betting on either side of a game. Since point spread betting requires that bettor to risk $11 to win $10, when betting is equal the bookie collects $11 from losers, pays $10 to winners, and pockets a tidy 10% profit from the vig. However, when the betting is unbalanced, the sports book "takes a position" on the game, and can either win big or lose big. There is a huge academic literature indicating that point spreads are efficient in that on average the spread is equal to the expected point difference on games, but this does not mean that betting volumes are equal on games.
There are a lot of bad teams in the NFL this year. According to this article, the Vegas books are unable to set lines big enough to attract equal betting on games involving the dregs of the league (Lions, Rams, Bucs, Chiefs, Redskins, Titans, etc.), leading to big losses.