Seems like the Evernote folks have written something especially for this case:
- Sign into Google Notebook
- Click on a notebook that you want to export
- Click the Export link at the bottom of the page
- Choose ‘Atom’ from the export options.
- Save the Google Atom Notebook file to your desktop
Then go over to https://www.evernote.com/GoogleNotebooks.action and just import the created "Atom".
Repeat for all notebooks.