Metacity has some hidden configuration to bind keyboard shortcuts to arbitrary commands desktop wide.
To access this configuration, open a terminal and run gconf-editor.
In the application the appears, use the folder navigation on the left to navitage to /apps/metacity/global_keybindings. Then in the right pane, look for entries named "run_command_N". The Value field of these entries can be edited to specify keyboard combinations.
Clicking on one of these entries will show some documentation in the lower pane. In the field, you can describe a key shortcut using their special format. Keyboard modifiers like Control, Alt, and the windows key can be expressed using <Control>, <Alt>, and <Super> respectivelly.
Once you have set up a key shortcut for a particular number, siwtch to the folder /apps/metacity/keybinding_commands. In this folder, edit the "command_N" entry with the corresponding number with the command you'd like to have run when your keybinding is pressed.