Apparently I missed this when it happened (closest date reference I’ve found is 29-September-2010), but I just learned that Google SideWiki is indeed available for Google Chrome. The SideWiki is just about the only reason why I’ve installed Google Toolbar on my Firefox.
The SideWiki for Chrome is a separate extension living on its own as a small icon next to the Address field, so there’s no need for the rest of the Toolbar (which isn’t available for Chrome yet anyway). All action goes via that icon, which shows a number if there are any SideWiki entries for the page that you’re on. You don’t get the sidebar, rather a dropdown from the icon, but other than that it seems to be fully functional.
Find and install here: google.com/sidewiki (use Chrome – in other browsers this link will take you to the Google Toolbar installation).