As others are reporting, Skype clients have been disconnecting and reconnecting around the world. Here is Boston, I've been off line and back again at least four times in the past hour. And when I've reconnect, an amazing small number of others are seen as on-line:
In recent weeks, I've been seeing over 9 million users on line at this time of day, so 615K suggests very little of the global Skype network is accessibile to me, if they are on-line at all. A few minutes ago, Jan was seeing 773K other users from his site in Malaysia, so this really is global.
We've known, at least since 2004, that Skype's peer-to-peer network wasn't strictly P2P. The vast majority of traffic (control and media) is P2P, but everytime a client comes on line (well at least at startup and each login), it interrogates the Skype Login Server at skype.com. We also know that a Skype client must establish a connection to a super node to successfully login. But I don't know if there is anything about supernodes that cause them to crash if they can't reach a centralized or semi-centralized Skype server.
It will be interesting to see how this develops. Hopefully Skype will be forthcoming, but if not, I'm sure third parties will piece together an answer.