Both of the Webkit-based browsers are inconsistent between operating systems as to their z-index layout. Likewise, Opera with their own layout engine is inconsistent between OSes. Firefox with its Gekko engine turns out to be the only one that renders the z-index visually identically, even though I was using Firefox 3 on the Mac and Firefox 4 Beta on Windows. I am very interested to know why the code got the intended results on all browsers on OSX, whereas 3/4 of the browsers which are on both OSes failed to perform the same on Windows.
*1 The page renders in Quirks mode by default, but using IE9's developer tools to force the page to render using IE9 HTML5 and CSS3 compliance mode didn't fix the problem. I didn't look at the page source code to see what's going on, but Dreamweaver code should be relatively clean, if not perfectly standards compliant.
Original discussion: http://getsatisfaction.com/livelabs/topics/keeping_a_photosynth_image_as_background