http://www.zdnet.com/article/skype-ditched-peer-to-peer-supe...

> ...the Skype peer-to-peer network architecture elected certain nodes to be "supernodes", to help maintain the index of peers as well as handle parts of the NAT [network address translation]/firewall traversal for other peers. This election algorithm chose only machines with open Internet connectivity, substantial uptime, and which were running the latest version of our peer-to-peer code.

I probably misunderstood the first part of his comment to mean that they also wanted to distribute some of their processing to clients.