 Complete instructions
Here's a step-by-step guide to fix the problem I was having in Tiger. While the Leopard users above had the same problem, the solution in my case was completely different.
(1) Download and install the .pkg as you would any other application.
(2) When the installer tries to add the prefpane and you're told it already exists, click CANCEL. Do NOT let the installer try to overwrite the file, as it will never actually copy the .prefpane file to the PreferencePanes folder. Only CANCELING the action copies the file. (Crazy!)
(3) Go to ~/Library/PreferencePanes.
(4) Right- or control-click the file Zimbra.prefPane. Choose "Show Package Contents" from the contexual menu.
(5) Navigate to the Contents folder inside Zimbra.prefPane. Open the file Info.plist in your favorite text editor. (Don't use a rich text editor. Use something like TextMate, Smultron, or BBEdit.)
(6) Go to line #14. Change the text com.apple.prefpanel to com.zimbra.prefpanel.
(7) Save the file. You may need to authenticate yourself because it's a system file.
(8) Go back to ~/Library/PreferencePanes and double-click on Zimbra.prefPane. This will properly add the prefpane.
(9) Be glorious.  | 
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