Harmonix is notifying those with accounts on their websites of a security compromise, saying they've disabled all passwords while they investigate the extent of the breach, and suggesting a precautionary change for those who are using their Harmonix password on other sites:

We're sending you this message because you have an account on RockBand.com, DanceCentral.com, and/or Creators.RockBand.com. We are investigating an apparent attack on our servers that we detected this week.

The security of your Harmonix user information is very important to us. We’ve taken the sites down while we investigate this incident and determine how our systems and the information we maintain may have been compromised. At this time, we have not found that any of our users’ information has been published or misused. None of our sites maintain any credit card information, social security numbers, or financial account numbers for any of our users.

As a precaution, we have disabled your Harmonix password. We’ll send you a notification when the sites are back up. When you log back in, we will require all users to reset their passwords. If you have been using your old Harmonix password on other sites, we recommend that you change those passwords too.