[NOTE: A couple of weeks after posting this, Bruno Amaral wrote an Apple Script that does a far better job of it. Check the comments for his script and instructions. Thanks, Bruno!]
Here’s a very simple Automator script that extracts the speakers notes from a Keynote file and writes them into a text file on your desktop. The text file it makes is very ugly, and I’m sure someone better at this than I am (= everyone) could have Automator do some cleanup of it.
To install it, download it (duh), unzip it, and then put it into MacHD/your_account/Library/Services (where MacHD and your_account reflect the actual names of your setup).
To run it, in Finder select the name of the Keynote file, and right click for the context menu. Go to Services > extract_keynote_notes.