As of this writing, the iOS and Mac App Stores are down, as well as the iBook and iTunes Stores. Yikes.
Update: Apple has isssued a statement on the matter:
BREAKING: Apple apologizes to users for outage that took down iTunes & other services. http://t.co/61NJWHH7pY pic.twitter.com/yZGQCRudYl
— CNBC Now (@CNBCnow) March 11, 2015