I think that I'll take away the following from this discussion:
1) Per the rules, the attacker describes his method of attack (magical or nonmagical) as he sees fit, within the bounds set by the rules and tempered by common sense. If he wants the attack to be non-lethal, he should expect to be able to make a compelling case for the lethality or lack thereof.
2) The group is not required to agree with the attacker's assessment of the attack's lethality. This decision will be influenced heavily by the type of game the group wants to play.
3) If the decision is that the attack description is reasonable, then the attacker retains narrative control over a take-out, and even concessions must be limited by the description.
4) If the group disagrees with the non-lethality of the attack as described, then the attacker has the option to revise his attack. If he goes ahead with the lethal attack, then he does so with the understanding that the GM might choose a lethal concession, if he feels that best suits the situation and the campaign. Note that just because an attack *could* be lethal or even should *likely* be lethal, doesn't mean that it *must* be.
I think that these guidelines allow for most of the viewpoints expressed earlier in the thread, but rely more on a group consensus as to how to handle lethality, rather that defining a specific formula that *must* be applied. Technically they represent more of a framework by which the RAW can be applied rather than new mechanics.
Some (occasionally silly) examples of my views regarding adjudication:
Ex: The attacker wants to incapacitate someone non-lethally by wrapping a grenade in bubble-wrap and inserting it into the opponent's mouth. The group respectfully disagrees that this constitutes a non-lethal attack.
Ex: The attacker wants to cast a telekinetic spell to knock his target out by hitting him in the head with a chair, noting that his character has the aspect "I've been in many a brawl in my time" and therefore has experience in such matters. The group agrees, and decides that it would probably be reasonable even for someone without such experience to wield a chair non-lethally.
Ex: The attacker wants to use a force spell to shatter a window and drive a cloud of glass shards -- non-lethally -- at his foe. The group notes that he is using a force 12 spell and decides that it really isn't possible for a chaotic cloud of razor-sharp shards driven by that amount of force to be used in a deliberately non-lethal manner, and that if the spell inflicts more than double the stress necessary to take the foe out, then death will result. The attacker decided to rethink the attack.
Ex: The attacker wants to shoot a foe's kneecap out with his sniper rifle from his concealed position on a nearby roof. He has the maneuver-generated aspect "Plenty of time to aim" and the character aspect "I was the most decorated sniper in my outfit, back in the war..." The players decide that he's eminently qualified to ensure a shot that is not immediately lethal ... but note that the target will probably need some medical attention fairly quickly.