Today is Pi Day, a day celebrated by mathematicians, scientists and geeks due to it being 3.14 (March 14th). The number Pi is about 3.1415926535 and thus on March 14th, it is celebrated as a special day every year.
Google had a special Pi Day logo last year but not this year. Here is a picture of the logo from last year.
The picture above is our theme for Pi Day 2011.
Google also did not have a logo for the 2009 Pi Day event.
I should also add it was Einsten's birthday today. He was born today in 1879, 132 years ago - yes, on Pi Day.