Thanks for all the replies everyone. Haven't had a lot of time to look over this thread in detail over the past couple of days, but I am getting to it.
Would it be fair to say that the "classical" explanation is that of kiriya mentioned above - i.e that the Arahants cetana/sankhara are karmically neutral? I'd really like to have the classical position down as a benchmark to evaluate the various ideas and suttas put forward in this thread.
Thanks for all the help.
"If beings knew, as I know, the results of giving & sharing, they would not eat without having given, nor would the stain of miserliness overcome their minds. Even if it were their last bite, their last mouthful, they would not eat without having shared."