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.