Professor Donald Tobin has a wonderful article giving some insights on the taxability of Ann Romney's participation in dressage (with some nice examples of how the passive loss limitation rules apply):
Ann Romney’s love of horses and Steven Colbert’s infatuation with Rafalca, one of her dressage horses, have created a buzz about horses, money, and taxes. Romney owns a one-third interest in Rafalca, and Rafalca will be competing, with her rider, Jan Ebeling, in the Olympic dressage event. In the most recent uproar, the Romneys are criticized for deducting $77,731 for the Romney’s share of Rafalca’s expenses. But here is the catch: Because of anti-abuse provisions contained in the Tax Code the Romney’s only actually deducted $49 on their return. Assuming the Romney’s are in the 35% tax bracket, the benefit to the Romneys was about $17. Not much worth working yourself into a lather about.