Gabe Newell has written a letter informing Steam users with the ongoing investigation of last year's security breach in November. Though the apparent hackers were able to access Steam's database, there's still no indication that information was stolen. However, in light of some new information, he asks the Steam community to remain vigilant:
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.
In compliance with certain state laws, some Steam users will be receiving a formal notice of this update.