They are saying the service will remain active for the time being but closing new sign-ups will surely doom the product and force them to shut it down eventually.
From Google's Blog:
At Google, we're constantly working to innovate and improve our products so people can easily find and manage information. At times though, we have to decide where to focus our efforts and which technologies we expect will yield the most benefit to users in the long run.
Starting next week, 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.
They then go on to suggest other products at Google that have similar bits and pieces of functionality as Notebook... but none of them will do what I currently use Notebook for.
