I think the main thing is that it just works. You push the button and you get your URL, no matter whether you're behind NAT, router or (probably even) firewall.
It would be interesting to know about such a capability of computers behind the router, because it would imply that the router is not an effective firewall - which I had assumed it is. Although to be fair, I suppose you can usually disable uPnP in the router (which should be the default imo).
I'm just guessing here, but I'd say it is based on a similar scheme as skype - the browser opens an initial route to the opera server, which essentially stays open (think ajax) while the browser is running, and then the server can route the requests back to this link. Any NAT is taken care of in the initial connection, so as long as it isn't broken, any "incoming" request will in fact be a reply to the original open connection.
OK, if they do that, it would work - but then it all would depend on the Opera server. If I create my own applications, I don't want to depend on some Opera service being up and running.
At the very least there would have to be some open source application that I could install on my own server.
If you have a UPnP capable router, it does allow apps behind the firewall to switch on forwarding for ports they need. It's often even enabled by default: There was some exploit recently that used that mechanism.
