The Steam user forums were reportedly hacked over the weekend, and as of the time of writing they're still down. A message appeared redirecting users to a hacker forum, which may be involved in the attack. Since the hack occurred, one Steam forum user claims to have gotten an e-mail from the hacker group.
A picture of the hack was captured on Reddit (via Kotaku), but the forums themselves are down for maintenance. The message on the forums plugged the hacking site Fkn0wned.com, and one user on NeoGAF claims he was sent a spam e-mail promising hacks, tutorials, porn, and giveaways from Fk0wned.
It's important to note that Steam's forums are a distinct login from the Steam service itself. If you've never taken part in the forums and don't have an account, not to worry. Steam forums users may want to change their passwords to be on the safe side, however, along with anyone who used similar passwords between the forums and their main Steam account.
Shacknews has contacted Valve regarding the reported hacks, and will update as more information becomes available.