The most important reason we know the revenge isn't acceptable is because Fortunato doesn't see it coming.  When we study this story, I always pose this question to my students, "What could Fortunato have done to Montressor to deserve these actions."  Students will come up with a great list of stuff, but the thing that really ties them together is that if Fortunato had done any of them, he would know that Montressor would be mad at him.

We've all accidentally made one of our own friends or acquaintances mad at us without knowing what we said or did would make them mad, but whatever happened didn't justify getting killed for it.  That's what makes Montressor's actions so unreasonable.  What, exactly, could Fortunato have done that was so horrible, but not even know it made Montressor mad??

I would have to say that, no, the revenge of the story is not acceptable for many reasons.  Perhaps the biggest reason that the revenge here is so unacceptable is that we, the readers, are given no clear cut reason for the revenge.  Montresor was wronged by Fortunato, no specifics, just "wronged"; it seems a little difficult to sympathize with Montresor, a murderer, or accept his actions, when there is nothing to go on.  For example, if Fortunato had harmed someone Montresor  loved out of some evil plot or plan to cause him pain, then you might be able to understand the revenge; however, even though understandable, it is still not necessarily acceptable.  Basically, nothing makes revenge acceptable, but in some cases, revenge is more understandable than others.

I would say certainly not.  Murder can never be jusitfied (especially one as brutual and inhumane as this one.)  Furthermore, we never know exactly what Fortunato has done to so enrage the narrator.  The best we can glean from the narrative is that he is a wine snob and something of a bore.  If this criteria justified murder, a good percentage of Dallas (where I live!) would be snuffed out!  :)