Natively Silverlight do not support 3D display and so I have got this question in head for a while "Which tech use Photosynth to display 3D things in Silverlight ?"
I have found a piece of anwer on a blog about Deep Zoom, a subpart of Seadragon. "Photosynth goes Silverlight"
We can read the name of the person in charge of the silverlight port "Mark Dawson" and also we know the name of the framework that have been used and surely modified : Kit3D
Since the project Kit3D seems to not have been modified for a long time.. I think they have incorporate it in their framework... and maintain it as closed source.