We contacted Microsoft to see if it's aware of this rash of hacks. The publisher sent Joystiq a statement saying "We do not have any evidence the Xbox LIVE service has been compromised," and indicating that these unauthorized logins have only occurred for "a limited number of members," with whom MS is working directly. The full statement is after the break.
Statement from Microsoft on unauthorized XBL account use:
"We do not have any evidence the Xbox LIVE service has been compromised. We take the security of our service seriously and work on an ongoing basis to improve it against evolving threats. However, a limited number of members have contacted us regarding unauthorized access to their accounts by outside individuals. We are working with our impacted members directly to resolve any unauthorized changes to their accounts. As always, we highly recommend our members follow the Xbox LIVE Account Security guidance provided at http://www.xbox.com/security to protect your account."