From a quick skim of the literature (and complexity zoo), there doesn't seem to be a non-deterministic version of PP. Is there a reason for this (e.g. PP=non-deterministic PP?)
Edit: Perhaps I phrased the question poorly: I was thinking more along the lines of BPP's relation to MA? Is there an equivalent version of an interactive proof system where Merlin needs only convince Arthur with probability >1/2?