Many users (myself included) have noticed to their dismay, that the Recent Items menu of the native Mac applications such as Preview and Text Edit are empty no matter how many times they are opened.  I had to choose the wording of this posts’s title carefully because the current theory seems to indicate that the upgrade to Snow Leopard didn’t cause the missing Recent Items menu, but that the problem had already existed, and the upgrade merely revealed it.   Regardless of the reason, the Recent Items menu can be restored by dragging the file com.apple.recentitems.plist to the Trash, emptying the Trash, then rebooting.  The com.apple.recentitems.plist can be found in Users/yourname/Library/Preferences

Move com.apple.recentitems.plist to the Trash

After you reboot, the Recent Items list should be populated again the next time you open files.  I’d also like to add that it’s a good idea to be careful when using third-party Cache Cleaners, perhaps to the point of not using them at all.  I’m pretty sure that this is how I broke my preferences.  In most cases, the Macintosh OS has built-in cache cleaners and maintenance tools that will operate correctly, and that are fully supported.