SideWiki is an interesting idea - it kind of takes the web back to the original two way conversation that the designers had in mind - with the caveats two large caveats:

• You won't see any SideWiki comments unless you have the new Google Toolbar installed
• The comments reside on Google, not on the site being commented on

Kind of a two edged sword - it's another place for comments to pop up that you'll need to monitor, and you don't have any ability to moderate the conversation that's going on about your site. I'm not sure what I think about it yet.