Skype's peer to peer underpinnings are one of those things that everybody knows about, but underneath the surface, Microsoft has made changes to the system; while it's still peer to peer based, it's backed up with, of all things, Linux supernodes. Immunity Security's Kostya Kortchinsky tells Ars Technica that he discovered the change about two months ago while probing Skype for security vulnerabilities; where before you would have seen many Skype "supernodes", made up of regular user systems that happened to have optimal speed connections in the right places, he saw instead a set block of around 10,000 supernodes that all appeared to be hosted by Microsoft. To make things just that little bit more interesting, they're all apparently Linux boxes, although that may tie into Skype's history of running some of its own servers on that platform.
At first the reports were met with some incredulity, until Ars sourced a quote from Microsoft which essentially backs the report up:
As part of our ongoing commitment to continually improve the Skype user experience, we developed supernodes which can be located on dedicated servers within secure datacentres. This has not changed the underlying nature of Skype’s peer-to-peer (P2P) architecture, in which supernodes simply allow users to find one another (calls do not pass through supernodes). We believe this approach has immediate performance, scalability and availability benefits for the hundreds of millions of users that make up the Skype community.
So in other words, it's now a bolstered system that still uses peer to peer, but has a more robust backbone. At least in theory; all of this sits in the shadow of Skype's recent IP discovery flaw.