Here are some relevant rulings we have:
1. If you are stunned during a move action, such as by a wall attack, you will be unable to complete the move.
2. If you are stunned during an attack action, such as by a damage barrier or counterstrike, you will be unable to perform a battle fury attack or sweeping attack.
Those are both specific examples and not really sufficient to draw a generalized conclusion from. I don't believe there are any other official rulings really pertinent to such a condition/effect being applied mid-action, but I could be wrong. It is obvious from these that incapacitate will affect an action in progress, but we're just not sure exactly how it affects it. When is the latest it could be applied to have an effect?
For example, we may know that applying it before the "Move Step" will prevent you from changing zones with a move action, but if you applied it between the "Move Step" and the "Enter Zone Step" then it doesn't really make sense for it to have any effect. Traps and stuff should still go off, because you DID change zones. But the wording we have could be interpreted that way. Rules Quote: "If a creature becomes unable to move during the move action, it stops the move sequence at that point, and the rest of the move action is canceled." So we know that being unable to "take a move action" counts as being unable to move and will stop a move mid-action. We know that it can't affect steps that have already occurred, so can't stop you from moving if you've already moved, but WILL cancel any steps still remaining. So, if the move action is still in progress...and it is able to stop it in progress...then that means it could be used to stop the Enter Zone Step and therefore stop traps from going off even though you have moved? That CAN'T be right and I don't think anyone would play it that way.
That was kind of a ridiculous example, meant to show how hard it is to draw conclusions from the tidbits of information we have. We know it can and DOES affect actions mid-process. But we don't know how. It will stop sweeping and battle fury attacks. Is that because they have a separate declare attack step? We don't know. Will it stop doublestrike or triplestrike attacks? We don't know. Will it stop a single attack currently in process? We don't know. When's the latest it could be applied to stop any of these? Even with Sweeping and Battle Fury, we don't know because the specific ruling was only regarding damage barriers and counterstrikes applying it. What if it got applied outside of those steps? Not covered. I think most of us would agree they'd be stopped as long as it was applied before their declare step, at least, but even that is making assumptions and drawing conclusions off of very limited information, which I hope I demonstrated with my movement example can be kind of a rocky road to travel.
I personally like the answer that someone suggested that it work similar to the rules for changing range/target. If it happens before the Resolve Spell Step, or before the Damage and Effects Step, then it cancels the action. Add into that if it happens before the Move Step of a Move Action. But I can't really give the supporting evidence needed to back this up. It's just what I like, and is no more or less valid than other interpretations. I think those saying it should only stop actions that haven't yet been declared run into a problem with the pre-establishment of it stopping Move Actions mid-move, and I don't like that such a ruling would leave additional strikes from Doublestrike and Triplestrike unaffected, as those don't have a declare step. But, I admit that their opinion is valid and is a sensible interpretation of the wording "can't take x action." In the end, we just still need an official ruling.
It is 4 in the morning here. I apologize for rambling and any potential errors I may have made while sleep deprived. I've been kind of avoiding diving into this topic because it irks me, as it does all of you, that we don't really have an answer. Hopefully this rant helps everyone to understand where things are coming from and where we're at, but the answer is still a very unsatisfactory "waiting on an answer."
