The code I demonstrated (with a bunch of fixes) is now available at https://github.com/DamianEdwards/dotnetConf2014
Principal Program Manager for ASP.NET at Microsoft.
@undefined:for SignalR, we've sucessfully connected >100,000 concurrent clients to a single box. This is idle connections. I'm currently working on profiling and load testing SignalR with active connections and hope to have more numbers soon. FWIW, supporing lots of concurrent idle connections is pretty easy in IIS/ASP.NET if they're hitting asynchronous handlers and you tweak some settings. You'll only be limited by memory in real terms. Once you start adding traffic is when it gets really interesting