http://www.splashdamage.com/forums/showthread.php?t=23194&page=2
Unfortunately, there are many cases where backstabbing is used where there is no particular misconduct on the part of the victim. If insta-kill backstabs can be used in open combat, it opens up a large jar of worms. Two people trying to manoeuvre behind one another at high speed can just feel a bit weird and disorienting. Weird-and-disorienting followed up by instant death is pretty svcky. You need to understand why you've been killed, or it feels like an act of God.
We did try some other things early on (essentially an anti-camper melee kill, where you have to remain in the "shadow" of the oblivious opponent before you're allowed to instant kill), but it's kind of over-working the principle: being behind someone who is standing still and focussed in the wrong direction is practically already a kill for free. You don't really need an extra move for it. We just like the idea that even in that situation, there's a bit of fight-back for the victim - a slim chance that they might get out of the situation they've offered up to would-be assassins..