OK, I hate to give them a free plug, but this web site is definitely worth checking out: Anybirthday.com Birthdate Search Engine. You can search for anyone's birthday (in the USA), and about half the time you'll find it. (You'll also get the person's zip code, which you can look up using one of these links: Census, ZipInfo, ZipFind.) Supposedly this data comes from "public records" ... but I have my doubts.