I hadn't installed ARD, as I run Tiger and it's included. But I had recently enabled the built-in VNC server on the ARD panel. Since I don't own Pacifist, and I had no intention of spending money on an app to use once, I wasn't going to easily get the pane from my OS X install DVD.
So what I did was download the ARD client update from Apple's Remote Desktop page. After running this, my Sharing prefpane magically re-appeared in System Preferences panel.

