UPDATE: Millions Potentially Affected By Steam Security Breach

UPDATE: Steam returned online this evening for users and released this statement to media outlets, alleging nothing unauthorized on their servers occurred from the breach:

Steam is back up and running without any known issues.  As a result of a configuration change earlier today, a caching issue allowed some users to randomly see pages generated for other users for a period of less than an hour. This issue has since been resolved. We believe no unauthorized actions were allowed on accounts beyond the viewing of cached page information and no additional action is required by users.

We are following an evolving and quickly developing situation where it appears millions of who use the Steam have been Scrooged by some sort of breach.
Users who logged into the service today were given access to another random users’ account information, including buying history, personal details, and partial payment information.
Unofficial sources have pointed to it being a caching problem. No matter: With personal details being exposed in some cases and accounts being charged in error, this could be one of the largest security breaches in some time, and certainly the largest to impact the video game industry. Well over 100 million people are active on Steam.
A statement a Steam community moderator claims this is not a malicious hack:

We’ve gotten reports that people sometimes see other people’s account information on the account page. Valve has been made aware of this and are working on a fix.
Some frequently asked questions:
– No, Steam is not hacked
– Creditcard info and phone numbers are, as required by law, censored and not visible to users

We will continue to monitor the situation. If you are on Steam, we recommend checking your banking, Paypal, and credit card information for any unwanted charges.