Copied from the Google Notebook Blog
"Starting next week [Jan. 19, 2009] , we plan to stop active development on Google Notebook. This means we'll no longer be adding features or offer Notebook for new users. But don't fret, we'll continue to maintain service for those of you who've already signed up. As part of this plan, however, we will no longer support the Notebook Extension, but as always users who have already signed up will continue to have access to their data via the web interface at http://www.google.com/notebook."