SHOW SOME L4D <3

Not much of a surprise this is a possibility, the two games do use the same engine after all. But even before Left 4 Dead was launched someone had already modded it (probably the demo) Here's nine minutes — reloading! — of Left 4 Dead gunplay on TF2's Dustbowl map (slightly customized).

Video from Kotaku here: http://kotaku.com/5089572/play-left-4-dead-on-team-fortress-2-maps