Yeah, with a name/handle like Anonymous0wn3r they sound very trustworthy. If they claimed they had a hand in something, it must be true.
Not sure if those tweets were actually from before it happened.
* GoDaddy: 10:35 AM - 10 Sep 12
* AnonymousOwn3r: 11:57 AM - 10 Sep 12
So, AnonymousOwn3r does not seem to have announced the attack before it happened.
Links to tweets: