Most of the worldwide Skype network has been down for a day now and it still has not recovered.
Skype's official word is that the problem was caused by "a deficiency in an algorithm within Skype networking software that controls the interaction between the user's own Skype client and the rest of the Skype network". Our own internal contacts within Skype also say that this was not a DDoS attack or anything else like that.
Then again: Skype's main development unit is in Estonia. Estonia's infrastructure was targeted by massive denial-of-service attacks earlier this year. This tied together with the fact that a new Denial-of-Service exploit against Skype server software was posted to securitylab.ru just hours ago has created lots rumors about what's really going on.
The exploit is quite simple and causes Skype client software to generate a large amount of calls, freezing the server it's connected to – and causing a reconnect to another server.