For all we know it could be running on an Ubuntu server using Mono. They had Miguel DeIcaza on their podcast once and said they would make sure it can run on Linux too.
It's not. StackOverflow and StackExchange run on a Windows stack. Their blog has a lot of neat information on how they set the entire architecture up.
And the whole point of Mono is to be able to run the windows stack on Unix environments. Hence my comment that "it could be running on Ubuntu using mono" :-)