Valve's Steam forums were taken offline today after a potential hacker intrusion. The hack manifested itself in the form of a “Notices” banner advertising cheat community Fkn0wned. Reddit users also reported spam emails from Fkn0wned or redirects toward the community's forum board. However, this was also soon taken down, replaced with a notice denying Fkn0wned's participation in the hack.
Steam forum users shouldn't be too worried, as forum details are separate from Steam account or password information (unless you've posted those details, of course, which would have been silly). Thankfully, there's no evidence that users' Steam accounts have been hacked just yet. However, when the forum comes back online, it would be sensible to change your password and check what information you're sharing, just in case. Fkn0wned's forum users are downplaying the attack, suggesting the work of a lone nut rather than any organized conspiracy.
Valve has made it through the hack-heavy 2011 so far largely unharmed, even as competitors such as Bethesda, Nintendo, Codemasters, Epic Games, and of course Sony saw high-profile intrusions call their security into question. Earlier this year the company announced Steam Guard, a service intended to offer users an added level of protection against potential breaches. If you've not registered for Steam Guard protection, this might be an excellent time to rectify that.