I happened to hear about the outage last night when writing an essay, so I went back to gameinformer to check, and here it is;
http://www.gameinformer.com/b/news/archive/2013/04/14/xbox-live-currently-encountering-sign-in-issues.aspx
Apparently, this was some routine maintenance, though I think it's safe to assume things haven't gone as planned, as although the service is up, many parts of it don't work right now, and also, when there was some maintenance with planned downtime, microsoft usually would alert people prior to the date.
They have also went on record saying that claims of a hacking were false.
It is, of course, a possibility that they are lying, but as of yet, nothing is proven.