
Just navigate to the Content » Resources folder of the program you want to edit then, find, select and drag and drop the keyedobjects.nib you need to edit to some other non-compiled .nib. The original compiled keyedobjects.nib will then open and allow every usual interface tweaking.
Of course, I had some concerns about this trick. But I used it to edit the Keynote 3 resources, and so far, I've haven't seen any problems -- and Keynote is as stable as before. A nice way to edit the Leopard interface, maybe?
[robg adds: I haven't tested this one.]