Blanco, I tried your sample, and the ai would not shoot over the wall. I tried moving the setpos commands from the inits to a trigger, and that did not help either. I have ARMA 1.08.
I have had decent luck with stacking "Obstacle - Saddle" objects (which is 2 meter long .13 meter high cement block) into a wall (5 or 6 high). It looks decent (basic cement wall), and the AI will shoot over it. Attached is a sample if anyone is interested. You need EditorUpdate_v102 for this.
A picture is also attached.
Note that you can CreateVehicle EditorUpdate_v102 objects, and then stack them, so that you don't actually have to have the addon in your missions. But I don't have time to build that into this example right now.
Edit: Note that AI (and player) can "hop" over these stacked object walls when they are in the AIs move path (unlike a fence, which the AI will never go over. I think hopping over a 1 meter high wall is actually desirable though... It would look stupid though to have AI hopping over a 2 meter high wall...
Edit 6/21/07: Uploaded revised test mission where enemy units have been "reveal"ed to the ai behind the walls. Seems to make them shoot over wall more often.
Edit 6/24/07: Revised test mission again to setdammage the Obstacle Saddle objects to 1. Re-uploaded it to attachment below (per ducus mucus input). Now they fire on targets 100 meters out.