Here's Google's take on it:
Because doodles are such a fun part of the search experience, we thought we’d share a fun little way Google will help celebrate your birthday. When you include your date of birth on your Google profile, you may notice a special treat on the Google homepage on your birthday (be sure to sign in). Click on the doodle for another birthday surprise!A nice little touch for those of you who have a detailed Google profile.
So, check it out. And then come back and tell me what you see. In true tin foil hat form, I keep my Google profile devoid of my personal info. Too visible to marketers and the general populous. As a true geek though, I have to admit that I considered adding my birthdate just so I could see this in action.
(Source: Official Google Blog)