Ok, I managed to figure it out:
1. You have to open the GConf Editor (gconf-editor).
2. Navigate to the /apps/metacity/global_keybindings key
3. There will be several values of the form: run_command_n (n = 1,2,3...12).
4. Edit the value of one of them to the keybinding you want (e.g., <Ctrl>1)
5. Go to the /apps/metacity/keybinding_commands key
6. Edit the value of the corresponding command_n to the command you want to run (e.g., gnome-terminal).
Perhaps there's an easier way to do this?