Where is it referenced? The only thing I can think of is using a birthday/date as a password and someone cracking/guessing that.

It has to do with the "birthday paradox" and is a kind of brute-force attack. Its involves both cryptography and mathematics.  It gets its name from the interesting result that more than likely  two or more people in a group of 23 share the same birthday is greater than 50%. As the size of the group increases so does the probability.  It can be an effective attack against some things. If I remember correctly it could take several hundred years to crack MD5 with a birthday attack. Its appealing because it's extremely easy to change document formats. Pretty much every document has some place where you can insert  “hidden data”.

For an audio explanation of a birthday attack, Steve Gibson did a nice job explaining it in a Security Now podcast a while back...episode #35.  Acutally the whole series that he did on crypto was very good (not to mention various other topics).

http://www.grc.com/SecurityNow.htm

-M