They are stored in /home/user/.calendar/db
You need to use sqlite to view them, but it's not a simple task. You'd have to first get the id of the note you want to extract. You can get plain text from the Components table if you just look at the Description field, or you can hit the Customproperties table Value field to get the html

If you only had one note:
QOrganizerNote in QtMobility seems to be the clean way of getting ahold of these entries, someone could cook up an extraction app easily enough