Unless I'm missing it, I don't see where you can add tags to product notes, but it would be useful to be able to do so (as long as those tags fed into a way to search or display content based on the tags).
You can now add tags to notes and whiteboards! The Tags field that is already available on features and other record types has been added at the bottom of workspace documents, as well. Try it out, and let us know if you have any feedback or additional ideas.