I've written my own birthday reminder module (
jahkids.com/joomla/mod_birthdays.zip) and I thought about adding that kind of functionality....
But you've got a problem, cuz you have to do your emailing from a process that is run once every day. I can't do it from the usual page generation process cuz I can't be sure the site will get a visitor every day.
...and anyway...is an automated email really that nice to get on your birthday