@LarryLarsen: I've actually experienced the same thing on a number of sites using IE 11.
I asked about it on Photosynth's forum and Henri Astre from the Photosynth team told me:
"Are you using IE11?
There is a bug in IE11 which is preventing an iframe to run in IE11 mode (thus with WebGL) when the parent page is rendered in a lower mode (IE10 for ex).
The iframe page is rendered with the rendering mode of the parent page.
So this is why an iframe (embed) can be rendered with the mp4 fallback video but clicking on the logo to view the synth will use the webgl viewer (as the view page on photosynth.net is rendered using IE11 mode).