Steam was taken offline by Valve this evening due to security concerns surrounding a potentially major data leak.
It appears as though user information has been randomly revealed to people using the service, with the potentially sensitive information of others being shown in place of people's personal details. Purchase histories, Paypal account names, and the amount of money in your Steam wallet were among the details reportedly on display.
It is being widely reported by Steam users that random account information has been made available, and at this stage there's no clear indication as to the scale of the problem. It's also not clear how long personal information was left exposed for.
In response to the incident Valve shut down their store front, in the process putting a hold on their big end of year sale. The company doesn't appear to have made an official statement at the time of writing, although the store seems to be back up and running now.