Just to put that into perspective, right now, you can buy off the shelf from Tibco an appliance for routing 10 million messages/sec (at an average message size of 50 bytes).
What Twitter does is not hard, and they do it badly.
Having used Tibco and having written high-performance message routing services for investment banks, I'd like to say yes it is hard, and Tibco does it badly.
What Twitter does is not hard, and they do it badly.