What OS is used by Google currently?

I love Google and naturally I like to know more about it. I was wondering what OS they use on their machines? Or on their servers?

I surfed the net but I could not find an official statement! :(
It is said that they use a customized version of Debian Linux. Cool! We, as lovers, have many things in common! ;)

Check the links and share your idea please:

• http://www.itworld.com/040618googledebian
• http://news.netcraft.com
• http://uptime.netcraft.com/up/graph?site=www.google.com