Right. Well, unfortunately in this day and age running 3D in a browser still requires a plugin.
If they turn away when they see that they need to install Silverlight then they must not care about trying Photosynth at all. Silverlight is a small download and the installer only has two buttons: "Install now" and "Finish". I don't think that's too complicated.
As for being able to include synths in pages (due to wide availability) Silverlight 2 runs in Internet Explorer 7 & 8, Firefox 2 & 3, Safari 4, Chrome, and sometimes even in Opera 10 on Windows xp, Vista, and 7.
It also runs on Intel Macs in Safari 3 & 4, and Firefox 2 & 3. I haven't tested it in any other browsers on MacOSX, but it wouldn't surprise me to find that it worked in other browsers there as well.
Also, on Linux, Novell is going to launch Moonlight 2 this September which will run Silverlight 2.0 applications like Photosynth. I've only tested the BETA in Firefox but synths crash it so far.