Going through Slashdot used to be part of a daily routine some time ago. Anyways, I had gone through this interesting article on Slashdot's data center setup back in October, 2007. It was certainly an interesting read, since it is good to know what it takes to setup and administer a pretty heavy website.
The interesting part was reading about what software they would be running on the Web Servers. As expected, all of their 16 servers run Linux, but RedHat 9 (really???...well, I guess this was setup back in 1999). I guess the distribution doesn't really matter as long as you can manage and upgrade whenever necessary.
In addition, they have 7 databases, running CentOS.
I am not an administrator, but the second part of the article did get into details about the Apache setup on the 16 servers which could be interesting to read.