On evented application servers, you can integrate with the event loop API.
And, I'm not 100% sure whether the spec allows this, but it would appear that on application servers that are not evented, you can even offload the socket to a thread running an event loop. For example Phusion Passenger's implementation allows this.
Could it, for example, replace a COMET server that is maintaining 100 connections (all doing long polling)? 1000? 10000?
Could it also be hacked into doing Websockets upgrade (and subsequently maintaining a large number of concurrent websockets conns)?
For example, while this API allows you to implement the WebSocket protocol, Nginx's input buffering will interfere with it.