The Internet knows no boundaries, and neither do the fraudsters looking to take advantage of others. Microsoft is investigating possible fraud taking place on the Xbox live service, after many customers have complained that their accounts have been stolen. On top of just getting the account name itself hijacked, some are claiming even more sensitive information is being lifted:

“Literally the next day my girl's account was locked out,” Finisterre wrote in an e-mail Tuesday. “I received a message on my Xbox that said: 'We are sorry we must log you out of Xbox Live because someone else is using your Gamertag.'” Other troubled users report that their credit card information was stolen to buy Microsoft Points, a form of currency used to by games downloads on Xbox Live.
Microsoft isn't looking into “specific” concerns just yet, which means they probably aren't dealing with those that claim they have been defrauded, but are doing a general search into what could be causing the claims. Quoting them, “security is a top priority for Xbox live”. It should be doubly so now, since soon they will be bringing PCs into the fold. Hopefully they'll come up with a better answer than “Call your credit card company” soon.