On 24 Feb 2009, at 17:14, Stefan Monnier wrote:

>>    (define-key (copy-keymap (current-global-map)) (kbd "M-c")  
>> 'ignore)
>
> This creates a new keymap (of same content as the global map),  
> modifies
> it and then throws it away.

Which is why changing the copy of the keymap should have no effect -  
but it does.  I reproduce on Emacs 22.

Note that

   (define-key (copy-keymap (current-global-map)) (kbd "x") 'ignore)

works as expected, i.e. it does nothing.

"x" is in my global key-map,  while M-c isn't bound there.

I also tried something like

(setcdr  (copy-keymap (current-global-map)) '(foo))

which doesn't do anything to the global keymap either, so failing to  
copy the tail isn't the problem.

Further, note that the M-c binding doesn't show up in the original  
global map after the define-key call above; however, the binding  
becomes effective.