All right, let's play the lefties' game and come up with a conspiracy theory, except what I'm about to propose at least has some valid points to make it seem credible.
Kerry has admitted to committing war crimes in Vietnam, so it can be said that he has a history of violence, mayhem, and criminal activity. He has something to gain by having the parent of a murdered American speak out against President Bush. Michael Berg is a known member or supporter of International A.N.S.W.E.R., so could logically be assumed to be either supportive of Kerry's campaign or hostile to Bush's. Kerry said he spoke with Berg, but wouldn't say when or regarding what. These are all facts or logical assumptions.
Therefore, I'm proposing a theory that Kerry conspired (possibly with Michael Berg) to have Nick Berg murdered. I know its in bad taste, but it is more logical than anything the left comes up with.