This is not the type of news you ever want to see, but it has happened. The Steam database has been hacked, with the intruders taking user names, hashed and salted passwords, game purchases, email addresses, billing address, and encrypted credit card information. Earlier this week, the Steam forums were taken offline after a hack, but now that hack went a lot deeper than previously thought. Valve founder Gabe Newell sent out a message stating just how deep the hack went, and it did compromise the database. Currently, Valve does not believe the encrypted credit card numbers or personal identity information was taken, nor that the protection used for credit card numbers and passwords was cracked. Valve is still investigating, so more information will be revealed later.
There is no evidence of fradulent credit card charges right now, but still, you should watch your account closely regardless. Some Steam forum accounts were hacked, and when the forums return, all users will be required to change their passwords. No Steam accounts have been hacked, since that information is separate from the forums, but still, if your forum password matches your account password, you should probably change it. Valve will reopen the forums later, but right now those will remain closed.
Valve and Gabe Newell are deeply sorry this all happened, and hopefully everyone's personal information and credit card numbers remain secure.