It's to commemorate the birth of the French amateur mathmatician Pierre de Fermat, he of the famous theorem. The logo is a green chalkboard with the relevant equation laid out. Now before you all start thinking: 'Blimey, that Garion's a bit clever' , I should say that if you hit the 'I'm feeling lucky' button without entering any search text it'll explain any Google logo..

..but I did know anyway of course, 'cos I AM actually rilly, rilly brilliant..