if they add a long animation for it, like one hand shuts the mouth and the nose of the enemy to supress his scream(you want silence right? try stabbing a needle to your finger..) while the other stabs the neck/spine etc... maybe then it would be acceptable. but the fact that silent kills takes the enemy (ai in our case) for a fool is not realistic at all. IF the possibility of enemy countering your attack would be included; like some kind of brawl.. then that would feel solid imho.
but anyway, what are we, ninjas? is this world of sam fisher? ..come on guys, let other games be games and let arma stay as simulation. (and let ace mod make that sim perfect :])
BUT! i would enjoy seeing melee action without silence. it would be great for mods too..
p.s.: i know OP does not mention ninja skillz, i'm just telling to "stealty" fellas :]

it's available in ace mod @arma2, and it is very useful. so no reason to leave it to mods this time..