Google Notebook was previously a "Google Labs" project, but it has left that phase to become an official service without the familiar "beta" label. Though the interface isn't extremely different, it has been nicely polished and finally looks like a finished product.
The old interface has a different logo, the navigation on the left is different, and several other things that have changed. Here is a screenshot of what it used to look like:
The changes are nice, and it looks like something I could potentially find a use for, but I'm still not rushing to start using it on a regular basis. Here is a screenshot of the new interface: