Though Marissa Mayer announced that Google Notebook will be launched next week, Erica Joy, another Google Employee, has already uploaded screenshots of Google Notebook on her Flickr account.
Erica is also saying that Google Notebook will also be accessible via an extension available both for Firefox and IE. The extension adds a right click menu option to note whatever page you're on. You can also highlight some text and note it as well.
Source: Google Notebook Screenshots | Erica Baker