Not much of a surprise this is a possibility, the two games do use the same game after all. But Left 4 Dead hasn't even launched and someone's already modding it (probably the demo, anyway.) Here's nine minutes — reloading! — of Left 4 Dead gunplay on TF2's Dustbowl map (slightly customised). Warning, the volume on this sucker is hecka loud for some reason, with tons of — reloading! — gunfire.
Left 4 Dead Meets Team Fortress 2's Dustbowl [CS-Nation, thanks AproposOfEverything]