I don't know if this has been said yet but with Vick(possibly), RG3 and Tony Romo in our division zone defense is a good thing because in zone the CBs eyes are looking at the QB where in man the CB will have his back to the QB(makes it more difficult for a QB to be able to run and pickup huge chunks of yardage).
Steelers play a ton of C3.
The seahawks are one of the few teams with a top defense that run mostly man coverage. And that is by necessity because of how they setup their front 7. I can link to an article on that if anyone wants to read.