@darkmobius, if you delete Chrome's cookies, do you still get this? I've tried to duplicate the error in Chrome on four different computers now and got it once, but it went away as soon as I refreshed and wouldn't come back.
I went ahead and tested in Opera as well and got it every time on the same machines, for what it's worth, though.
Firefox 3: Launches Synther
IE8: Launches Synther
Safari 5: Launches Synther
Chrome 5: Launches Synther
Opera: Unable to detect that Photosynth is installed
On the other hand, you don't actually need to visit the site in order to launch the synther, since it's a separate app anyway. You can just make a desktop or taskbar shortcut or use the Start menu.