I hate to sound like a broken record, but if you want a look at where Photosynth and Bing Maps are headed in the future, keep tabs on Bing's upcoming Read/Write World project. There haven't been any announcements yet in 2012, but they talked publicly about it all during 2011.
I've collected every public talk that I know of at http://bit.ly/readwriteworld
They, too, are using WebGL (as well as CSS3:3D Transforms, HTML5 Canvas, and possibly even Silverlight 5 for browsers like Internet Explorer which don't yet support WebGL).
As to transitions between images being far smoother than Photosynth, I guess all I can say is that you've probably only used Photosynth's Silverlight viewer, which is nowhere near as smooth as the original 2006/2008 Direct3D Photosynth viewer. Hopefully moving to WebGL, Silverlight 5, and CSS3:3D (all of which use your computer's GPU like Direct3D) should result in smoother graphics.