I was very presently surprised when I found this doodle on my Google homepage this Sunday, which was my birthday.
Got a lot of queries about this one as I put the picture on Facebook and so wanted to clear all doubts and answer the queries.
1) No, it was not visible to everyone in the world, however much I would have loved if that was the case! This was a personalised logo, visible only to me, only on my birthday.
2) No, Google did not do this for me because I am an employee. We do have a few lucrative perks, this is not one of them. Any Google user can get this Doodle on their birthday, and here's how:
a) Go to your Google Profile
b) Update your birthday and you're done!
If you go to the Google home page on your birthday (when signed into your Google account) you should also find the customised Google Doodle, with the little mouse over "Happy Birthday [firstname]"
Go ahead, get your own Google birthday doodle.
P.S: This was announced in this Google Blog Post.