Jack, I can't remember who said it but someone did say a ground game was not needed in todays NFL. Words to the effect it was passe.
I've said something similar to that, though not exactly. The correct way to phrase my outlook isn't that a running game isn't needed, but rather, "A strong rushing attack is not correlated with making the post season or post season success."
What you need out of your running game varies a great deal. The Packers don't need as much as the 49ers, for example. 3.5 YPC would be perfectly fine for this offense. Just enough to get defenses out of Tampa-2 or Cover-2 all game would be plenty. The last few games have been nice. Hopefully this gives everyone else just enough to think about so they can't sit in a shell all game.