Quoted from fireball2:Negatory. Who is a subject, whom is an object. Who is back? is grammatically correct because who is the subject of the sentence. You would use whom for things like, "This is for whom?" or "To whom am I speaking?"
And it would be "for Fuck's sake." FYI It is Fuck whose sake we are bemoaning.
Wow. Both points are 100% grammatically correct here. Well done sir.