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]