I'm not convinced. It would use every trick in the book to punch holes; stun, upnp, sip, it's own protocol on top of udp, it would even send data over port 80 or 443. And if that failed Skype would use a third Skype client to open a port, or even route the calls through the third party.

I always thought this was the major reason why Skype was so successful. It would penetrate almost any workplace firewall and "just worked" for most people. Shady, yes, but it worked.