Roblem

PS with stackoverflow getting about 1.5 mill unique visitors a day the performance seems pretty good. It would be nice to know how many servers they have in their farm/clowd.

Stackoverflow is actually pretty open about their server infrastructure
http://blog.stackoverflow.com/2010/01/stack-overflow-network-configuration/ 

http://blog.stackoverflow.com/2008/09/what-was-stack-overflow-built-with/