We were looking to get in some Team Fortress 2 action tomorrow, but Valve has put the kibosh on our plans. Starting tomorrow at 6pm California time, the Steam service will go through some scheduled software maintenance on the servers. The company expects the downtime to last no more than three hours with a scheduled end time of 9pm.
Just like when the PlayStation Network and Xbox Live perform routine maintenance, all online aspects of the Steam service will unavailable during this period of time, as well as the Steam Community, downloading games, and making purchases in the Steam store. Other areas of the service that do not rely on then servers to operate and games that do not require matchmaking should be unaffected.