This week Valve Corporation head Gabe Newell has announced that the gaming hub Steam has suffered an attack on their forums which may well have put credit card numbers and other users personal information at risk. The attack apparently went down this Sunday and Valve is currently letting everyone who had any personal info listed at Steam to watch their credit card activity in the near future for possible fraudulent activity. Here after the attack the Steam Forums are closed and will remain closed for the time being while Steam itself will remain operating as normal.
Note that the attack itself was not necessarily aimed at the credit card information and other personal bits of users at Steam, but that the larger attack did have the potential to access said information. A note from Newell read something to the effect of don't be scared, but also keep your guard up. Have a look at a couple bits of that statement here:
"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. We are still investigating. We don't have evidence of credit card misuse at this time. Nonetheless you should watch your credit card activity and statements closely. [The database in question] contained information including user names, hashed and salted passwords, game purchases, email addresses, billing addresses and encrypted credit card information [but] we do not know of any compromised Steam accounts, so we are not planning to force a change of Steam account passwords (which are separate from forum passwords). However, it wouldn't be a bad idea to change that as well, especially if it is the same as your Steam forum account password." - Newell
Good advice when such big bits of information are at risk. I'd guess a call to your credit card company noting the possibility of fraud wouldn't be a bad idea also. Meanwhile if you're feeling just fine and dandy because your credit card seems fine to you, you can continue playing your games on Steam all night long, as only the forums are currently down until they fix it all up.