Silverlight 5 requires the user to explicitly grant each domain permission to use the 3D features.
1) Go to the URL above.
2) Right click on the plugin area
3) Pick "Sliverlight"
4) Go to the "Permissions" tab
5) For the domain you are currently on pick "allow" for "3D Graphics: use blocked display drivers"
No idea if they have un-blocked display drivers.
Indeed, I haven't yet found a driver that wasn't blacklisted by Silverlight 5.
I have updated my bug report:
I am actually happy that they blacklist a lot of drivers (all of them? I don't know): this adds pressure on driver vendors to implement what we need (resilience to DOS, etc). WebGL and Silverlight have the same needs in this respect.
Also, as far as I can tell, 3D is only supported on Windows. Mac and Linux are out of luck.