Skype has officially acknowledged the problem but hasn't provided much information about the cause. It should supposedly be resolved in a few hours. "Some of you may have problems signing into Skype -- we're investigating, and we're sorry for the disruption to your conversations," the company said three hours ago via its Twitter account.
Interestingly, Skype's status monitor (heartbeat.skype.com) doesn't show any problems. All services are said to be working normally, with the last known issue dated Thursday, December 16. On that day, Skype reported "sign in and registration" issues among other difficulties, but it's unclear whether there is any correlation with today's outage.
Last month, Skype reached a milestone of 25 million simultaneous users. Many of those people rely on the service for work and other important duties, so the outage is undoubtedly a major inconvenience. The last major interruption occurred in 2007 and took 12-24 hours to resolve. As of writing, I remained unable to connect to the service.
Update: Skype said the issue stems from a number of "supernodes" being taken down. Supernodes supposedly act like phone directories for Skype and there are usually plenty available. Today, many were taken offline by a problem affecting some versions of Skype, so the service may appear offline for those users.
"Our engineers are creating new 'mega-supernodes' as fast as they can, which should gradually return things to normal. This may take a few hours, and we sincerely apologise for the disruption to your conversations. Some features, like group video calling, may take longer to return to normal," the company explained.