Steam is planning hardware updates today. Because of that everyone can explace to see downtime at some point during the day. This means that a few people might actually see sunlight today as well. Here are all the details Valve provided.
We'll be updating some of the Steam servers on Monday, June 18th, 2012 at approximately 15:00 PDT (GMT-0700) and we should be finished by 15:30 PDT.
We expect all users to be affected, but do not expect all users to be affected at the same time. Chat, game lobbies, and the community web site will be affected for a set of users for a portion of the maintenance. Some content downloads may be briefly interrupted. Game play should generally not be interrupted but may be impacted for some multi-player titles.