Documenting this would be great, since it caught me by surprise and took 
a while to figure out.

To get the Cmd module to work, atm you have to override the call to 
parse_and_bind, setting self.completekey isn't enough, so that one is 
broken on Leopard.

Patching the readline module is indeed a lot of work, you need to 
convert readline names like 'Tab' to integers to pass them to libedit, 
and convert  the syntax as well. It would be better if libedit would 
just accept readline syntax, but that's outside the scope of Python.