@ Nate Where are you getting this? I'm going with a second term POTUS dying scenario, natural causes...political assassinations give me the willies. So, the POTUS keels over right before the ceremony. At what point, previous to that, is he not POTUS? As I understand it, a second term is a seamless continuation of the first term. At no point between election and moment of inauguration is the POTUS not the POTUS. The same would be true of the VPOTUS. There would be a seam between a POTUS dying and a VPOTUS taking the reigns, but no bigger seam than is written into the Constitution.

You're right, I think, for a POTUS- elect. Same scenario, first term POTUS- elect dropping in the moments before the ceremony. I'm pretty sure, since the VOPOTUS- elect hasn't been sworn in, that at the moment of power change the Speaker would be the nominal Head of State.