What's the current state of internet switches? Back when I used to run the Napster backend, one of our biggest problems was that switches, regardless of whether or not they claimed "line-speed" networking, would blow up once you pumped too many pps at them. We went through every single piece of equipment Cisco sold (all the way to having two fully loaded 12K BFRs) and still had issues.

Mind you, this was partially because of the specifics of our system - a couple million logged in users with tens of thousands of users logging in every second pushing large file lists, a widely used chat system which meant lots of tiny packets, a very large number of searches (small packets coming in, small to large going out) and a huge number of users that were on dialup fragmenting packets to heck (tiny MTUs!).

I imagine a lot of the kind of systems you'd want 10M simultaneous connections for would hit similar situations (games and chat for instance) though I'm not sure I'd want to (I can't imagine power knocking out the machine or an upgrade and having all 10 million users auto-reconnect at once).