I used to think AM Stereo was vaporware, that is until someone resurrected it on the internet, and I've heard of platform motion, like you're on a soundstage that's tilting from side to side. I may have heard that in a truck one time, when hearing a radio station doing that panning effect. I didn't know why it was happening or what band the radio was tuned. It was at night I think.
I have heard someone pushing the idea that HD receivers can pick up AM stereo, and also heard that Ibiquity was openly hostile to AM stereo. The HD radio getting analog AM stereo could be a side effect being that AM stereo is a form of Quadrature, and HD receiving quadrature formed signals, but I'd have to wonder if the sound quality would be any good, how close it would be between the two systems to be in any way compatible.
Bear