Deleted All Configuration Data to "Solve" Problem
I then re-installed all my extensions. I then restarted DW CS4 and had the damnable Memento back, but... then I also had the WebAssist preference check-box configuration menu Ray mentioned in his earlier post. I told Memento to never start, restarted DW, and all seems right for now.
I had originally downloaded the free PayPal assistant (which is too basic for real life). I uninstalled it and its version Memento and about the same time (later? earlier?) installed the components of the Web Designer suite. Maybe that's why I didn't have the WebAssist Preference check boxes?
But, WebAssist guys... this is truly unprofessional! Your Memento spamware is causing a lot of people to get DW crashes that cause hours of work to "fix". If you are going to put an application on MY machine that I did not ask for an do not want, please make it a polite player.