After upgrading to OS 10.6, I noticed that Microsoft Word and Excel were slow to launch. They would each go through the 'Optimizing Font Menu Performance' task every time they were launched; normally this is done only after the apps are installed or updated. The problem turned out to be duplicate fonts installed on the system, and this can be easily resolved using the Font Book application.
If any duplicate fonts are present, they will be identified by a yellow warning triangle in the Font column, and these can be safely disabled. The easy way to do that is to choose Edit » Select Duplicated Fonts (to select all duplicates) and then Edit » Resolve Duplicates (to disable the duplicates).
Note that Snow Leopard has changed font handling in many ways; that link contains detailed information on the changes.
[robg adds: You can disable duplicates in 10.5, too. I marked this one Snow Leopard Only due to the menu item that doesn't exist in 10.5, and it seems the duplicate fonts weren't causing a problem in 10.5.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091015044155906