According to Valve's co-founder Gabe Newell, the attack compromised the database which tracks users logins, passwords, game purchases, e-mail addresses, billing addresses and encrypted credit card information. Whether or not these hackers have the technical nous to decrypt the credit card information remains to be seen.
"We do not have evidence that encrypted credit card numbers or personally identifying information were taken by the intruders, or that the protection on credit card numbers or passwords was cracked." Said Newell. "Nonetheless you should watch your credit card activity and statements closely."
Steam is the latest in a line of recent attacks by hackers on gaming-related targets, starting with the downing of the PlayStation Network earlier this year.