I'd just like to be an advocate for Linux users. I'm sure the Mono/Moonlight team at Novell will implement Silverlight 3 compatibility as soon as possible, but for now, they are on the verge of Moonlight 2.0 final release.
I predicted this mismatch of Moonlight 2.0 release vs. the Photosynth Silverlight 3 viewer's release back when the Silverlight 2 viewer was promoted to the main viewer on the site. (See http://blogs.msdn.com/photosynth/archive/2009/04/09/New-Features-Highlights-CrossPlatform-Viewer.aspx#9571568 )
Perhaps you could sniff the user agent string of Linux machines and either serve them the Silverlight 2 viewer (and editor), or load the site as usual, but give them a choice to revert to the SL2 bits until Moonlight 3 is in a state of compatibility sufficient to run the new viewer and editor, as you offer the Direct3D viewer to Windows users.