I've just started looking into it, and as far as I can understand, you can't "push" a call to others. You will have to supply a web page and the javascript to connect to a "meeting room" or whatever (using ajax?) to "find" the other peer, and exchange connection parameters, before the webRTC stuff kicks in. (Correct me if I'm wrong)

Yeah, the "signalling" process is implemented by applications, and needs either a WebSocket or SSE stream.

So we could use a browser extension to listen in the background to a central phone server for incoming calls while we happily surf the web and then get the "ring" opening the url in a tab for the incoming call to take place?

Interesting...

Yeah, but why would you want to take calls in your browser? Why not a separate application (node-webkit, Firefox app) that runs as its own process a la Skype clone?

WebRTC is just a protocol for applications. So you'll need to have Hangouts or Skype or whatever client using WebRTC open, and then it should work when someone calls you through them, on the web.

Take Facebook Chat for example or whatever they are calling that integration with a Java Skype plugin. Instead of using that Java plugin, it could use WebRTC in the future, and it won't even need Skype. It could just work with Facebook users, which is something Facebook might want anyway, or at least until Skype uses WebRTC, too.

Before you can "ring" somebody in their browser, their presence has to get established first. So, they need to have signalled their presence to a signalling server first. That can happen via loading a specific URL, or being logged in to a specific Website. It's not possible to just magically know that they are around without them announcing it.