How to edit presenter notes in Keynote '09? I'm using Keynote '09 (I'm also interested by a solution working
with the last version of Keynote since I will upgrade if the
function is there). I'm making a lot of presenter notes.

With some of them I have prepared a set of relevant questions
the watchfull listeners might ask and some I may ask them.
I put all these in the presenter notes which I can look at
during a presentation.

How may I edit these notes, during a presentation, so as
to take into account good questions and answers coming either
from me, the speaker, or the audience?
Unfortunately, Keynote '09 can't achieve such a feature. You can, however, this feature has been added in the 2013 version of Keynote.

Enable the presenter view (in this example, I'm going to use the "Rehearse Slideshow" feature).

If you don't see the presenter notes, go to the top right corner of the screen and select the middle icon, then check off the "Presenter Notes" box.

Now that you see the presenter notes, it may not look like you can edit them at first, but if you slide your cursor to the top right corner of the notes box, the "Edit" button will appear. Click on it, and start typing (you can also edit it by double-clicking anywhere on the box).

Hope this helps.

(P.S.: For some reason, I'm unable to upload any pictures in order for me to show you better. Once StackExchange fixes those bugs, I'll update the answer with the appropriate pictures).