While Googling to answer this question for myself, I wasn't happy with the above reply and kept "at it" until I found the actual answer...
EACH custom keyboard shortcut you set via the Gnome menu Preferences -> Keyboard Shortcuts gets a directory and %gconf.xml file under the directory ~/.gconf/desktop/gnome/keybindings
I currently have 5 keybindings defined, so this is the result:
chris@myhouse ~/.gconf/desktop/gnome/keybindings$ ls
custom0 custom1 custom2 custom3 custom4 %gconf.xml
Best to tar these to backup and/or transfer to another computer. Which is something I'm in the process of doing.
ALSO, you might want to check out Autokey. http://code.google.com/p/autokey/
It is a daemon that runs and can interpret keypress combinations but has far more capabilities than Gnome Keyboard Shortcuts. I use Gnome Keyboard Shortcuts to launch programs and Autokey to do text insertion and & keypress manipulation.