There was a 1/23 patch. If you left your Steam settings alone I think you'll automagically download and install it if you restart Steam, or something like that.
Oh, wait, well I thought that but I just launched L4D to test it and it seems to be patching, so do that early if ya can, though it's a tiny patch.
January 23, 2009 - Left 4 Dead Update Released
Updates to Left 4 Dead have been released. The updates will be applied automatically when your Steam client is restarted. The specific changes include:
Left 4 Dead
Fixed an exploit with melee where swapping weapons ignored the intended delay between swings.
In Versus mode, the Tanks' health now reflects the damage he takes from burning.
Fixed an exploit related to the "joinclass" command
