It actually supports multiple senders connected to the same receiver simultaneously. Some apps already use this so you can control the receiver from different phones.
In the case that one client can be connected to the receiver (I don't know enough to know if this is a correct assumption), the receiver acts on content that is streamed to it. If other people playing the same game, pushed their interactions to a server that also is serving the receiver, the receiver's content would be updated with all the players interactions. Clients could be updated with pubsub socket connections, and the receiver has a stream from the server. I bet there is good potential for gaming with Chromecast.
afaik, the youtube, netflix, etc, apps actually connect to a server, not the device. so you could easily have all the devices connect to a webserver, only one would need to tell the comcast to start that webpage...
