Although I worry about the physical contact escalating to hand-to-hand combat, I do really like the idea of taking someone out without shouting or shooting someone, which basically alerts everyone where you are.
Making a close-to-real silent kill as possible has some interesting aspects—basically you get to continue killing more unnoticed. However, the opposite sucks; for instance, if you sneak around your opponents’ front and get directly behind them, your first shot is going to give you away, which kind of sucks.
So how do we get around avoiding physical contact, but still have a silent kill. Personally, if I was tapped on the foot or shoulder by an opponent that I did not know was there, I would just honorably give him or her the benefit of the doubt and fall silently dead for them. How I see this different than the knife idea proposed (which sounds fun, but I think could get out of hand) is that I would not (I think) react so aggressively. I am just out, no challenge to it…the person doesn’t/wouldn’t have to even tap me for a kill, just as long as they were three feet behind me without me knowing.