Never heard of it until I read this here:

Not only Cronkite and Rather participated in this appalling exercise [to defend the Warren Commission], so too did Eric Sevareid, appearing at the end of the last show and saying that there are always those who believe in conspiracies, whether it be about Yalta, China or Pearl Harbor. He then poured it on by saying some people still think Hitler is alive and concluding that it would be impossible to cover up the assassination of a President.

The Pearl Harbor conspiracy theory is pretty well known, but what is the one about the Yalta Conference? Or China for that matter? This is circa 1975.