The graphic above shows the likehood of covering by point spread, with the circle size proportional to the number of games with that line from 2000-2012. Overall, favored teams covered the spread 47 percent of the time, and this figure is relatively flat through a 16 point spread. After that, the chance of covering drops off precipitously, and teams covered just 30 percent of the time.
Credit to America's gambling public, though. Reports out Vegas make it sound like most people stayed away from the insane -28 point Broncos-Jags spread, although the money that did come in tilted towards Denver.