Google may have added this feature back in October, but I'm just now noticing it. Although I don't use footnotes very often, I know some folks that really like them. Alex at GoogleOS pointed out some issues with the new footnote system when they first hit the scene.
Somewhat related, the Google Docs team released a cool Gmail gadget in November. The new gadget allows you to access your Google Docs directly from Gmail. In fact, you can drag and drop a filename into your email and a link will be created automatically. If you're a die hard Google Docs user, this seems like a no-brainer. You can turn this feature on in the Gmail Labs settings.