Recently we learned that it is probable that the intruders obtained a copy of a backup file with information about Steam transactions between 2004 and 2008. This backup file contained user names, email addresses, encrypted billing addresses and encrypted credit card information. It did not include Steam passwords.
We do not have any evidence that the encrypted credit card numbers or billing addresses have been compromised. However as I said in November it’s a good idea to watch your credit card activity and statements. And of course keeping Steam Guard on is a good idea as well.
We are still investigating and working with law enforcement authorities. Some state laws require a more formal notice of this incident so some of you will get that notice, but we wanted to update everyone with this new information now.
via Source