Today I lost all my notes from the Notes gadget of the Vista Sidebar, which of course contained some very important stuff. After some research on the net it turns out that I'm not the only one who experienced this problem...
There is a way to get them back, but that depends on having the 'Previous Versions' turned on in Vista. If you have you can restore the settings.ini file that is in the following directory:
In this file all the settings of your active Sidebar gadgets are stored, including your notes.
In my case I could restore the file and after a restart of Sidebar.exe I got my notes back. So a good file to include in your back-up if you don't have the 'Previous Versions' option turned on.