I feel this is an interesting UI pattern as it is unintrusive & very effective. I guess web developers can use this idea to simplify navigating data intensive applications with such simple shortcuts & the Help layer.
Another cute idea out of the GMail Labs is the Forgotten Attachment Detector. When the feature is turned on, you'll get an alert ("It seems that you might have forgotten to attach files.Send this message without attachments?") if you mention attaching a file but forget to do so. I noticed it works if there is a line like "the file is attached" & you forget attaching the file but not if you have a sentence that includes "check the attachment".
Related links:
Google Reader - the online/offline Feed Reader