All of a sudden, I noticed that my Sharing preferences panel was missing from the System Preferences. The actual SharePref.prefpane did exist in my System folder, though, along with all my other system pref panes. After some Googling, I found a post on MacNN.com where someone said this happened after installing Apple Remote Desktop (ARD) client, as ARD actually modifies the prefpane with its own settings.
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.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051013211028500