If you have an account registered on Steam, you might want to read this.
Earlier, Valve discovered that a group had found their way into the Steam forums. Because of this, the company shut the forums down, and will issue a mandatory password update to its users upon its return. However, it was soon discovered that the hackers may have also accessed user info for Steam itself; credit card numbers, login info, and billing addresses may have been taken by the hackers.
As there is no concrete evidence that the info for Steam was taken, they are not issuing a mandatory password update for it. However, it is advised that users change their login info in order to avoid any potential harm. Valve is still investigating the situation.
Source: Steam Forums