Basically, since web browsers are finally getting some basic built in 3D capabilities, the Read/Write World team in Bing is wanting to write a single viewer in the new web standards like HTML5, CSS3, and WebGL which will open all of the different imagery formats that they care about, like photosynths, panoramas, 3D models, spin movies, streaming video, etc. and will open on any device whose browser supports HTML5.
Today, all of these media types have separate viewers in separate technologies on different devices. Creating a single viewer in web standards technologies (now that the standards are finally powerful enough to do basic 3D on their own) which doesn't require a special browser plugin such as Silverlight (which you can't get for game consoles, mobile phones, etc.) means that as long as your device has an HTML5 compliant browser, you should be good, once the Read/Write World viewer is up and running.