Steam Downtime Announcements has word on Steam downtime (thanks Shok). Word is: "Our data center's uninterruptible power supplies experienced a power failure. The power is back on now and we're working to get service restored as quickly as possible. We apologize for any inconvenience this may have caused." This is followed by: "We have made significant progress restoring availability, and service should return to normal soon. Update: The latest post on this indicates things are mostly fixed:

We are up and running at a normal user load. There are a couple of lingering issues which we will continue to work on until they are sorted.

Thank you for your patience during this time!