Today, at the LOGOS Colloquium, Carlos Moya (València) presented his views on how to defend the principle of of alternate possibilities (PAP) from Frankfurt-like cases, which he published as chapter 2 of his Moral Responsability (Routledge 2006).
In a nutshell, and if I didn't misunderstand his presentation (I haven't read the chapter), the main idea was the following one. John's being responsible for murdering Smith doesn't contradict PAP, for John could have done otherwise after all: he could have merely involuntarily killed Smith.
(Carlos originally stated this in terms of unintentionally killing Smith, but as issued in discussion with Prades, the notion of intentional action in place cannot be merely that of action appropriately caused by beliefs/desires, and Carlos replied he was happy rephrase it in terms of (in)voluntary action.)
I worried, in connection with Jose's, that this seemed to be dangerously close to the following (unsatisfactory, I take it) general way of dispelling any possible counterexample to PAP: if the agent is responsible, s/he could always have done otherwise, for s/he could always have done the "corresponding" thing without being responsible. It was hard for me to see how the sense in which the act of murdering and the act of involuntary killing someone (in the Frankfurt situation) were "different actions" could fail to vindicate that same sense in the latter, trivializing case.