In the recent mailbag at Football Outsiders, Aaron Schatz ranks defenses on their ability to force their opponent into a three-and-out. The Green Bay Packers have a very good defense, so it's not a huge surprise that they're ranked high. But a high percentage doesn't necessarily correlate to a great defense. From Football Outsiders:
The rankings somewhat follow the rankings for the best defensive DVOA ratings, except that teams which get a lot of turnovers and sacks will rank lower on the frequency of three-and-outs. Pittsburgh, for example, was just average in three-and-outs per drive.
Football Outsiders ranked the Packers as the number two overall defense in 2010, and they were ranked fourth overall for three-and-outs: averaging them on 28% of opponents' drives.