Currently the best sources I can find are people's experiences until something official goes out:
Just got home and saw my Dad on his lunchbreak, told me he's checked his bank account and found that he's been charged for 6000 Microsoft Points under his gamertag. After cancelling the card etc. we checked xbox.com and found that his download history included something called a 'PREMIUM GOLD PACK' and his bank account listed having bought content for Fifa 12, a game neither he or I own.
This isn't much of a news thread, since nothing official's gone out, but it seems plenty of people in the above threads are experiencing the same thing. I advise you check your bank accounts and purchase history. My own account seems to be safe, but that doesn't mean all of us are.
Let's just hope this isn't another PSN incident.