Updates to the Source Engine and Windows Dedicated Servers have been released. The updates will be applied automatically when your Steam client is restarted. The specific changes include:

Source Engine

• Fixed Garry's Mod failing to spawn new entities (was failing due to "userinfo" command change)
  
• Fixed background music playing after joining a game if the game was launched via a desktop shortcut

Source/Half-Life 1 Dedicated Server

• (win32) Fixed "Steam Validation Rejected" error when connecting to a game server

News Source: Steam News