I think what you're saying about an opt-out for webmasters is the most satisfactory option. Keep in mind though, a lot of webmasters just setting up a website may like this sidewiki thing because it means they won't have to setup their own forums; they can just refer people to the google sidewiki.
I hate to say that having to install the google toolbar to use it is the biggest turn off for me. I hate unnecessary clutter.
Maybe the sidewiki would be a good thing for Mozilla to try and incorporate into
Snowl?
Dan.