A Google enthusiast has found an interesting hint about a new Google Drive app called Keep. It looks like a replacement for Google Notebook, that has been discontinued back in 2009.
There are multiple references to Google Keep in the GDrive code, including some URLs like: https://drive.google.com/keep/?note. In fact, the codename for Google Keep is "memento" and the MIME type for Google Keep files is "application/vnd.google-apps.note".
He also found the service's icon and a short URL http://g.co/keep that redirects to the Play Store page for a non-existent Google Keep app.
Google already has a Chrome extension called Scratchpad which syncs with Google Docs. Let's hope that Keep is better than both Scratchpad and Google Notebook.