from the SI article that Greg linked:
In the memo obtained by The Associated Press, the NFL reminded teams of league policy that calls on coaches to discourage the practice. There is no specific rule on the topic.
If the NFL really wants to control fake injuries they have a couple of alternatives.
1. Charge a timeout every single time any player is injured. If a team is out of times outs give their opponent 10 yards (not 5) for delay of game.
2. Let the play continue. If the defense has a player down they can play with 10 men. If he is down while over the LOS- offsides.
I know you don't want a player down with a neck injury like N Collins and have the next play run right over him. So I will add an amendment. If they player is carted off the field no time out will be charged.