Skyps is down cause the "Supernodes" (The Servers which save the contacts and handle the login) are not anymore able to handle the 4245621456215426456 different versions of skype.
The problems are mainly located in EU, Asia and USA.
Currently they try to get some servers which should act as "Mega-Supernodes" to make the basic skype stuff aviable again (chat and voice). THis should be solved in a few hours (they said:D)
Group conversations and other stuff can take a bit longer.
Also they said they will inform skypeusers via twitter if they got any news.