Xbox Live users should keep an eye on their accounts, and any credit card associated with that account, and especially take notice of any strange activity involving FIFA 11 or 12. Several users have complained, through multiple gaming publications, of hackers recovering their accounts to a different machine and using them for purchases associated with the soccer game.
Ars Technica reached one of the victims, whose experience is broadly representative of what's happening to others. The user received confirmation emails regarding Xbox Live purchases, investigated, and found FIFA 12 had been played on his account (with two achievements unlocked), and his credit card had been used to buy two large lots of Microsoft Points, which were then spent on FIFA 11 downloadable content.
Microsoft's response has been a boilerplate comment that, though it acknowledges "a limited number of members have contacted us regarding unauthorized access to their accounts," they do not have any evidence suggesting Xbox Live as a whole has been compromised. They say they are working with the hack victims to resolve unauthorized charges to their accounts. That generally involves locking down the account while Xbox Live security investigates.
There's no need to panic, but there is a need to immediately notify Microsoft if you spot unauthorized charges or activity on your account. That should be the case anyway.