I went through mine, and some of them are actually born on Jan 1 (according to their profile). But I'm not, and I get the birthday notice on both ID's that I use, but I don't appear on the "Today's Birthday" block.
I've double checked the criteria on the notice, and the only thing that is checked is "Today is User's Birthday" under date criteria, and "User is logged in" under User Criteria.
Very Strange.