I don't know how it happened, but my default Notes folder shows up as a
"Folder containing Mail and Post items" instead of a normal 'Notes' folder.
I can't see a way to change it back to IPM.Stickynotes either.

Any tips on where the misconfiguration is, and/or how to set it back?

The folder appears visually as a Notes folder in my Outlook 2007 UI, it's
mainly my sync tool that has trouble with this - it (Funambol) refuses to
sync it as a Notes folder since it appears to be for Mail and Post items.

Otherwise this really doesn't cause me any other problems - that I know about!