Thanks for your fast answer Shan :-)
I was also thinking about a workflow on note creation that would send an email to the case's owner.
But my first choice would have been to use the notification accelerator, build a view, and subscibe to the related RSS feed. I don't like the idea of spaming users with dozens of daily emails. Unfortunatly, this is not working for notes (this accelerator
works for all entities except the system ones).
I was searching for other ideas but it seems that basic solutions are often the best ones.