Pavarotti vs. The Walrus

So Google is celebrating Pavarotti's birthday today by linking him on the homepage, a custom I've quite enjoyed over the years on various birthdays and anniversaries of important people, events, yadda yadda (e.g. The Lunar Landing, Einstein's birthday,etc.) My question is: what are the criteria (if any) that Google uses to determine who and what gets commemorated in this manner?

I became curious, because John Lennon's birthday was a few days ago, and I had wondered why Google didn't acknowledge it, and now Pavarotti is being celebrated, and I'm thinking, hmm...is opera so much more culturally relevant than rock 'n' roll, particularly when "John Lennon" registers a million and a half more Google hits than "Pavarotti?" Clearly, Google can't possibly acknowledge every outstanding public figure or historical event, so there must be some sort of formula or process that determines who gets the goods.

I emailed Ube's man, who works at "the Google" (in the words of G.W.), to see if he knows the secret. Anyone out there know?

xo
Bree