Even though the saga of Michael Vick has been discussed by the media (as well as on the internet) ad nauseum, it's probably the best thing that's happened for questionable players like a Tank Johnson. They're seeing the repercussions of his actions and it's not pretty. And if watching Vick's life implode right before their very eyes isn't enough to influence these questionable players to turn their own lives around then I don't know what it would take to change them.
With that said, I think the timing is perfect to roll the dice and take a chance on a player like Tank Johnson.