Google Notebook provides options to export each notebook as HTML or in the Atom format, but it's not very easy to export all your data. A Greasemonkey script automates the tedious work by reformatting the mobile page and adding links to the HTML or Atom version of each notebook.
The default format is Atom and you should use it if you want to edit the notebooks on your computer and then import the files to Google Notebook. It's a good idea to use the HTML format for archiving your notebooks.
This script requires Firefox, Greasemonkey extension and a download manager like DownThemAll that saves all the files linked from a page or some of them.
After clicking on "Export notebooks", right-click and select "DownThemAll!". Choose a folder where to save the files, type nbid in the "Fast filtering" box and click on "Start".
Peter Shafer wrote a similar script for exporting Google Docs documents.