Worth a look at C10k (challenge to build a server capable of 10k concurrent connections) : http://www.kegel.com/c10k.html
and a resulting server software that steps towards that (powers youtube etc..) lighttpd fly light : http://www.lighttpd.net/story
Blogosphere posts on High Connection numbers to a web server
(Automatically generated External Blogposts of no relation to this blog)