Today 22 Dec'10 millions of Skype users faced difficulties signing in to Skype. The problem is caused by a large number of Skype "supernodes" taken offline as a result of a bug affecting some versions of Skype app. "Supernodes" are the key of Skype P2P functioning, these clients' machines are coordinators of P2P network. Normally the role of "supernode" is automatically dedicated to Skype clients  by some Skype algorithm, which takes into account computer resources and internet connection bandwidth. Details of the algorithm are not available.

Currently Skype engineers are creating 'mega supernodes' (according to Skype website) and hopefully will resolve the issue in a couple of hours.

More details on the official Skype page.
Skype twitter.