Brian, the current way that privacy is handled here on the Photosynth site is to simply set your synth|pano to 'Unlisted' in the site's synth|pano editor. To do this with any of your synths, sign into your account on the site, load any of your synths or panos, and click the [ Edit this Photosynth ] or [ Edit this Panorama ] button directly below the lower edge of the viewer.
Once in, on the first tab on the right edge of the editor, you'll see a setting for either public or unlisted.
Unlisted synths and panos can still be viewed by anyone who has a link to it (i.e. there is no password protection), however it will not be listed on your profile, in Bing Maps, in the 'Explore' page here on the site, or in search results, so it is quite safe unless someone you give the link to decides to send the link to the person you don't want to see it.
For more information, see: http://photosynth.net/faq.aspx#q2 and http://photosynth.net/privacy.aspx