The Steam Users' Forums highlight some latent MMO code found in the latest Team Fortress 2 patch (thanks Shacknews), indicating Valve has some further plans for persistence in the teamplay shooter, or, more likely, they are messing with us after other code discoveries inspired MMO TF2 jokes. Here's the code snippet:

// TF2 MMO
"TF_MMO_LFG" "Looking For Group"
"TF_MMO_Quest1" "You must kill %s1 %s2 to complete this quest."
"TF_MMO_Monster1" "Boars"
"TF_MMO_Monster2" "Rats"
"TF_MMO_Monster3" "Bats"
"TF_MMO_Monster4" "Wolves"
"TF_MMO_Monster5" "Spiders"