Well, I think there is no Thrash Metal or Death Metal concerts without mosh pits.
I think everybody knows that in every concert is possible to have mosh pits. If you want to avoid mosh pits, keep distance from the stage or keep very very close to the stage or simply go to another site when the mosh pit take place.
If you want to prevent mosh pits, I think people will begin to avoid Wacken Open Air. Imagine: the biggest metal festival, without mosh pits! This is insane!
So, all mosh pits, all over the world, have risks. The metalheads that uses to participate the mosh pits accept that risk and the metalheads that avoid it do it all the time; so there is no surprises here, nothing new.
This year I remember to see Soufly, and soon after the "no mosh pits please" sign in the big displays, Max Cavalera just said: "I want to see that mosh pit here!". I mean... he didn't saw the advice, and he consider it natural. So do I. Mosh pits are part of the metal world, and it's not up to you to forbid it, whatever the reasons.
Anyway! I think you should not try to forbid it. Maybe you can display some security warnings, but nothing more.
Till' Wacken 2011!! /,,/