So it's like the Birthdays within the Calendar, which takes them directly from Contacts (back when I didn't know this fact, I thought it was a leftover from a previous Google sync), but at least that can be turned off.
Well it's even worse on my side (i.e. double entries in the notification area), as my actual calendar is being synced with Outlook, which automatically makes birthday "appointments" from each contact that has the birthday field filled out.
I guess the old Outlook wisdom applies... "use the notes field for birthdays instead of the birthday field". :/