Joystiq reports: "Left 4 Dead has received an update on Xbox 360, which ... actually, we haven't a clue as to what it did. We've contacted Valve for details regarding the inoculation. Perhaps Valve's statistics have revealed a few nasty cheaters that need to be squished.
We really don't know, but we'll update when Valve gets back to us with exquisitely detailed patch notes. You've "refined the ambient shading algorithms?" Fantastic! Tell us more."