CNet has the scoop on what a Microsoft representative calls "reports of fraudulent activity and account theft taking place on the Xbox Live network." The company is investigating the claims, which include use of stored credit card information to run up massive Xbox Live points charges.
It's not yet clear how extensive these problems are, but anecdotal evidence from forums and web sites shows how the apparent theft can manifest itself. It's not yet known if the unauthorized use is the result of hacking in Microsoft's network or a phishing scam to get login details from individual users.
We're waiting to hear back from Microsoft on the extent of the problems and what specific steps are being taken to fix them. We'll keep you posted.
Update: Major Nelson: No Xbox Live security breach