Microsoft has been moving Skype from its original peer-to-peer architecture to a more centralized system for some time. After the Snowden shitstorm, critics have been implying that the move was NSA-related.
EDIT: This really seems like an interesting question: _are_ there any advantages an attacker would have with skype's centralized system that they wouldn't with their previous p2p system? From what we've seen so far, I think the differences (from an attacker's perspective) are trivial.