Neither Microsoft nor Sony have confirmed the nature of their problems, but as Polygon reported, a hacker group identifying itself as Lizard Squad had previously threatened to take down the services on Christmas day. The group is taking credit for the outages, though it promised to stop its attacks after being handed several hundred dollars' worthof gift cards from Megaupload founder Kim Dotcom. Lizard Squad quickly sent out a tweet offering support and promotion to Dotcom's latest venture.
But it's not yet clear how much involvement the group had in the downtime: some four hours after it said it had halted the attacks and "went dark," both PSN and Xbox Live services are still down. The group says the services are still suffering from the aftermath of its attack, but it's possible that the networks are just straining under the load of the new consoles gifted over the the holiday period.
Update December 26th: Story updated with Kim Dotcom's apparent involvement and continued downtime details.