Leopard limits the maximum number of Recent Places in the pop-up menu of the Open/Save dialog windows to a mere five items. Following this excellent tutorial that robg wrote on finding hidden preferences, I figured out that if there is a hidden preference somewhere, it must live in /System » Library » Frameworks » AppKit.framework » Versions » C » AppKit. Additionally, since nearly all apps in Leopard use the Cocoa Open/Save dialogs, it was obvious that the preference key likely follows the Cocoa naming scheme and begins with NSNav, just like the rest of the bunch.
And the winner is: NSNavRecentPlacesLimit. To apply, type the following in the Terminal, where 12 would be the number of recent places to display in the Open/Save dialogs:
defaults write .GlobalPreferences NSNavRecentPlacesLimit -int 12
Alternatively, you can also open ~/Library » Preferences » .GlobalPreferences.plist with your favorite plist editor, add a new key named NSNavRecentPlacesLimit and set an integer value of your choice. (Being not really the Terminal type of guy, this is the way I actually did it.) Once you have "collected" a larger list of places in the popup menu, you will also notice that they are sorted alphabetically, not chronologically, which would actually seem more logical given the term "recent".
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090106183327762