If I had to give it a percentage, after watching the footage a few times, I would say I am 99% confident that this was NOT a setup. It can easily be seen that Will Smith was laughing at the joke himself, before he caught sight of his wife taking it poorly. Just after that, the camera is off of them for a few moments, and then Will Smith is walking up, strikes Chris Rock, who, by all appearances, seems shaken - the rocky transition into trying to make light of it, Will Smith screaming at him to keep his wife's name out of Rock's mouth, and Chris Rock telling him somewhat tumultuously that he's going to. And then Smith's ridiculous words at acceptance of his award... pining for sympathy with tears, all the while talking about the character he played being a "fierce defender of family." No apology forthcoming, of course. The lack of any response by the establishment to remove Will Smith, etc. It all seems entirely too plausible - and who on Earth would be foolish enough to tarnish (or even give the HONEST impression that they had tarnished) the achievement of an award they had obviously been seeking? Why would Smith do this? Why wouldn't it be more obvious that it was a stunt? Do we think that Smith is "standing by" waiting for the right moment to inform us that it was all some social statement or experiment? Again... the plain witness of what went on as being the reality is simply too plausible.
And even if Smith did come forward, and stated that it was all a stunt, you'd likely be the type to then claim that their statement that it was a stunt or their reasons for claiming to have pulled the stunt, was a lie. Tell me I'm wrong.