If I am not mistaken we are getting close to the 48 hours line of the world major voip service provider failing to provide its service. Remember we are not only talking about (millions of...) 'free' users who pay nothing and enjoy the ride. Many Skype users I know have good American money invested in their SkypeOut accounts. They use it as a major international communication tool, Including for business. this is a very very problematic situation for Skype.
I have read many explanations by now. According to Valery Marchuk from SecurityLab.ru an exploit code published by an anonymous user on their forum may be the reason. Marchuk says the code uses standard Skype client to call to a specific number, causing a "denial of service of current Skype server and forces Skype to reconnect to another server.
David Beckemeyer posted a more reasonable yet much more amazing theory about the nature of Skype problem related to their usage of "Supernodes". According to Beckemeyer Skype depends on "supernodes": selected end user computers preforming as database servers and other network services. Thus, when the number of supernode skype clients goes under a certain minimum the network fails to preform. Beckemeyer think it could theoretically take days for this pathology to end.
By the time i finished writing this my Skype turned green. and then gray again. and then green again. we'll see in the morning.
More about Skype Supernodes