It's been a long time since I tried assigning a keystroke to a script using anything other than FastScripts or LaunchBar, so I took another look at doing this myself. Assigning a keystroke to a script using the Keyboard and Mouse preferences is still possible with Leopard-if the application puts up its own script menu, as is the case with apps like DEVONthink Pro and Word.
However, with the universal script menu (activated with AppleScript Utility), it's not that simple. Since you posted that it does not work for you, and since I'm not one to shrink away from a challenge, especially after I put my foot in my mouth, I've spent some (entirely too much) time exploring this. It is possible to activate the universal script menu using the keyboard, and this keystroke is set using the Keyboard and Mouse preference 'Move focus to status menus in the menu bar', which by default is set Ctrl-F8. You can then use the cursor key to navigate the menu, or type the first few letters, of the script, and type return to run the script. If one wanted to use this method, then prepending the name of the script with numbers would be a relativity fast way to select the desired script.
I also found that you can assign a keystroke to the script, and that shortcut will be active in the menu once the menu is activated with Ctrl-F8. However, the system will try to open the script in Script Editor rather than running the script, so there's no utility in doing this. The way to do this, just to prove it can be done, is to create the shortcut in 'All Applications' rather than selecting OmniFocus, and then you have to log-out and back in to see the shortcuts in the menu. For whatever reason, applying the shortcut to OmniFocus does not work, but as the keystroke opens the script rather than running it, it's a moot point.
Bottom line is FastScripts is the way to go-most of the features are available for free (Up to 10 shortcuts), and the registered version is only $15.