"twitter has servers dedicated to high-volume users without having to use urls/dns like that"

That's why they need so many load balancers w/ failover:

twitter.com has address 128.242.240.148 twitter.com has address 128.242.245.212 twitter.com has address 128.121.243.228

Load balancers aren't flexible to work with especially when dealing with sessions. I imagine Twitter's front end nodes are doing nothing but routing URLs to backends, which are doing the real work. So the front end balancer takes http://www.twitter.com/unixy and routes the actual server-side request internally (where scalability is implemented).