Joystiq is
reporting that a Steam user database containing "usernames, 'hashed and salted' passwords, transcripts of game purchases, email and billing addresses, and encrypted credit card info" has been hacked.
Valve currently does not have evidence that credit card numbers or "personally identifying information" was actually taken, though they are still investigating. Although, if they had access to billing addresses.... eeeehh.
Take the usual precautions here, if you're a Steam user. Change passwords, watch accounts, etc. Valve is the biggest in the business for what they do with Steam, and they've always seemed to be on the up-and-up so if they're saying things are safe I'm willing to believe them... but just be careful and attentive.