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It would be great to be able to drop 3-d models into these 3-d views. Is there an interface permitting me to do so?
To quickly answer your question, no, unfortunately there is no way to currently combine existing 3D geometric models with either the photosynths or spherical panoramas hosted on the Photosynth site.
In answer to your question about whether there is an equivalent to Sketch Up, traditionally Virtual Earth/Bing Maps 3D users have used one of two different programs:
1: 3DVia's model builder: http://www.bing.com/maps/Help/VE3DVIAInstall/VE3DVIADownload.aspx
2: Caligari's TrueSpace: http://www.caligari.com/downloads.html
As far as people being able to display these 3D models on Bing Maps, we are in a transitional period from using a Windows-only 3D globe which only worked in Firefox and Internet Explorer (this was discontinued 2010 November, though you can still access it by going to a site which is using the Bing Maps AJAX 6.3 map control, such as http://connect.garmin.com/explore and pressing the  key after clicking the map) to using Silverlight or HTML5 for Bing Maps.
Silverlight 4 does not have access to your computer's graphics card for 3D graphics (Silverlight 5 at the end of the year will) and CSS3's 3D capabilities (though much less powerful than what is offered by Flash 11, Silverlight 5, and WebGL) are still not standardized.
Because the old Bing Maps control only worked in two browsers and only on Windows and the newer technologies which work in all browsers on Windows and Macs do not currently have the 3D power necessary to do real 3D graphics, we're left in limbo.